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THE 


Physiology   of  the  Senses 


By  JOHN   GRAY   M'KENDRICK, 

M.D.,    LL.D.,    F.R.SS.L.    AND    E. 

PROFESSOR   OF    PHYSIOLOGY   IN   THE   UNIVERSITY   OF  GLASGOW 

and   WILLIAM    SNODGRASS, 

M.A.,    M.B.,    CM. 

MUIRHEAD   DEMONSTRATOR   OF    PHYSIOLOGY   IN   THE   UNIVERSITY   OF   GLASGOW 


WITH    127    ILLUSTRATIONS 


NEW  YORK 
CHARLES    SCRIBNER'S    SONS 

743  &  745  BROADWAY 
1893 

A II  rights  reserved 


GULIELMO  •  TENNANT  •  GAIRDNER 

MEDICINE  •  APUD  •  UNIVERSITATEM  •  GLASGUENSEM  •  PROFESSORI 

HUNC  •  LIBELLUM 

COLLEGA   •   COLLEGE 

DISCIPULUS    •    MAGISTRO 

AMANTISSIMO  •  UTERQUE  •  ANIMO 

DEDICAVERUNT 


AUTHORS'    PREFACE 

It  is  the  aim  of  this  book  to  give  a  succinct  account 
of  the  functions  of  the  organs  of  sense  as  these  are 
found  in  man  and  the  higher  animals.  The  Authors  have 
refrained  from  discussing  with  fulness  of  detail  either  the 
comparative  physiology  of  the  senses  or  the  numerous 
interesting  questions  of  a  psychological  character  that 
inevitably  arise  in  connection  with  the  study  of  the 
mechanism  of  sensory  perceptions.  Each  of  these  aspects 
of  the  subject  would  require  a  volume  for  itself.  On  the 
other  hand,  a  perusal  of  this  volume,  which  has  been 
written  so  as  to  be  readily  understood  even  by  those  who 
have  not  made  physiology  a  special  subject  of  study,  will 
be  a  suitable  preparation  for  entering  upon  the  more 
recondite  questions  that  underlie  physiological  psychology. 
The  Authors  have  endeavoured  to  treat  the  physiology  of 
the  senses  as  fully  as  space  would  allow,  and  have  also 
suggested  comparatively  simple  experiments  by  which 
any  one  interested  in  the  subject  may  test  some  of 
the  statements  for  himself.  They  would  also  direct 
attention  to  the  last  chapter,  in  which  an  attempt  is  made 
to  elucidate  the  nature  of  the  physiological  basis  of  sensa- 


viii  Physiology  of  the  Senses 

tion,  in  the  hope  that  it  may  be  found  to  be  a  contribution 
to  speculative  thought  on  this  problem. 

While  every  page  has  been  subjected  to  the  careful 
consideration  and  revision  of  both  Authors,  it  may  be 
mentioned  that  the  Introduction  and  the  sections  on  Sight 
and  Hearing  have  been  mainly  written  by  Dr.  Snodgrass. 

J.   G.   M. 

W.   S. 

University  of  Glasgow, 
March  1893. 


CONTENTS 


GENERAL   INTRODUCTION 
Sensory  Mechanism 


Terminal  organs  of  sensory  nerves 
Nerve  matter  and  nerves 
Nature  of  nerve  current 
Origin  of  nervous  system 
Structure  of  nerves  and  nerve  cells 


PAGE 

2 
3 

5 

7 

10 


Paths  of  Nervous  Impulses 


The  spinal  cord 

12 

The  medulla  oblongata          ..... 

19 

The  cerebellum     ....... 

19 

The  pons      ........ 

21 

The  cerebrum       ....... 

22 

Sensory  Centres  in  the  Cortex  of  the  Brain 


The  centre  for  vision     ..... 

.       30 

The  centre  for  hearing  ...... 

•       32 

The  centres  for  taste  and  smell 

•       34 

The  centre  for  touch      ..... 

•       34 

The  muscular  sense       ..... 

.         •         •       36 

Physiology  of  the  Senses 


Relation  of  Stimulus  and  Sensation 


Quality  of  sensation 
Quantity  of  sensation    . 
Sensations  and  perceptions    . 


36 

37 
39 


THE  SENSE  OF  TOUCH 


Structure  of  the  skin      .                   

41 

(1)  The  true  skin    ....... 

42 

(2)  The  epidermis  ....... 

42 

Structure  of  tactile  organs      ...... 

45 

(1)  Free  nerve-endings  ...... 

45 

(2)  Nerve-endings  in  corpuscles       .... 

■       45 

(3)  Nerve-endings  in  connection  with  tactile  hairs    . 

5o 

Nature  of  the  tactile  mechanism     ..... 

52 

Sensitiveness  of  the  skin         ...... 

54 

Sense  of  locality   ........ 

56 

Absolute  sensitiveness    ....... 

56 

Fusion  of  tactile  impressions ...... 

58 

After-tactile  impressions         ...... 

58 

Information  from  tactile  impressions       .... 

59 

Theories  as  to  touch      ....... 

62 

Sensations  of  temperature      ...... 

64 

Sensation  of  pain  .         .         .                   . 

67 

The  muscular  sense        ....... 

68 

THE  SENSE  OF  TASTE 


The  organs  of  taste 

Minute  structure  of  the  gustatory  organ 
Physical  causes  of  taste 
Physiological  conditions  of  taste     . 

Differentiation  of  tastes  . 

General  sensibility  of  the  tongue 

Subjective  tastes     . 

Nerves  of  the  tongue 


70 
71 

73 
74 
76 

78 

78 
78 


Contents 


XI 


THE  SENSE  OF  SMELL 


The  organs  of  smell       .... 
Physiological  anatomy  of  the  nose 
Physical  causes  of  smell 

Chemical  nature  of  odorous  substances 

Flowers  and  odours 

Odour  and  heat  absorption 

Odours  and  ozone 

Odours  and  surface  tension 
Special  physiology  of  smell    . 
Mode  of  excitation  of  the  olfactory  nerves 


PAGE 

80 
81 
86 

87 
89 
89 
90 
90 
9i 
93 


THE  SENSE  OF  SIGHT 
I. — Structure  of  the  Eye 


Coats  of  the  eyeball 
Contents  of  the  eyeball 
The  optic  nerve    . 
Movements  of  the  pupil 


97 
105 
109 
in 


II. — Physiology  of  Vision 

1. — Laws  of  Dioptrics 

The  physical  nature  of  light  . 

Reflection  and  refraction 

Action  of  lenses    ..... 

Formation  of  images  by  biconvex  lenses 

Spherical  aberration       . 

Chromatic  aberration     .... 

Optical  properties  of  a  system  of  lenses  . 


115 
116 
119 
120 
122 
124 
125 


2.  —  The  Dioptric  System  of  the  Eye 


Focal  points 
Principal  points 
Nodal  points 


128 
129 
129 


Xll 


Physiology  of  the  Senses 


3. — Anomalies  in  the  Eye  as  an  Optical  Instrument 


1.  Divergence  of  optic  from  visual  axis 

2.  Divergence  of  line  of  regard  from  line  of  vision 

3.  Chromatic  aberration        . 

4.  Spherical  aberration  . 

5.  Astigmatism     .         .         .         .         .         . 


PAGE 

131 
131 
131 
132 
132 


4. — Adjustment  of  the  Eye  for  different  Distances 

The  near  point  of  vision 

Irradiation    ..... 

Entoptic  phenomena 

Examination  of  the  interior  of  the  eye 

The  visual  angle  .... 

The  size  of  the  retinal  image 

The  blind  spot      .... 

Action  of  light  on  the  retina  . 

Amount  of  light  required  to  excite  the  retina 

Persistence  of  retinal  impressions  . 


137 
140 
141 

143 

145 
148 
149 
150 
152 
152 


5. — Sensation  of  Colour 


Complementary  colours 
Colour  as  dependent  on  the  r 
Colour  blindness   . 
Coloured  after-images    . 
Theories  of  colour  vision 


etina 


158 
158 

159 
161 
161 


6. — Binocular  Vision 

Movements  of  the  eye  .... 
The  ocular  muscles  .... 
How  an  object  is  seen  as  one  with  two  eyes 
Perception  of  solidity  .... 
The  stereoscope  ..... 
The  telestereoscope        .... 


170 
172 

i7S 

180 
181 

184 


Contents 


xni 


Estimation  of  distance 
Estimation  of  size 
Illusions  of  vision 


PAGE 

187 
190 
192 


SOUND  AND  HEARING 

The  external  ear    . 

External  meatus    . 

The  middle  ear 

The  Eustachian  tube 

The  chain  of  bones 

Movements  of  the  bones 

Response  of  the  tympanic  membrane  to  sound  waves 

Transmission  of  vibration  by  the  auditory  ossicles  . 


200 
202 
204 
207 
209 
211 
214 
218 


The  Internal  Ear 


The  osseous  labyrinth   . 

The  auditory  nerve 

The  membranous  labyrinth 

The  cochlea 

The  cochlear  canal 

The  organ  of  Corti 

The  inner  hair-cells 

The  outer  hair-cells 

Innervation  of  the  cochlea 


223 
225 
225 
228 
230 
231 
233 
235 
237 


Auditory  Sensations 


Physiological  characters  of  sounds 

240 

(1)  Pitch 

. 

242 

(2)  Intensity  or  loudness 

246 

(3)  Quality,  timbre,  or  klang  . 

. 

247 

Resonators    ...... 

. 

251 

Analysis  of  compound  tones  by  resonators 

. 

252 

Noise  ....... 

262 

General  mode  of  action  of  the  ear  . 

. 

263 

Analytic  power  of  the  ear 

269 

XIV 


Physiology  of  the  Senses 


The  Psychical  Elements  in  Auditory  Sensations 


Externality  of  sound      ..... 

•     277 

Direction  of  sound          ..... 

.     280 

Distance  of  the  source  of  sound 

.     281 

Memory  of  sound  .         ..... 

.     282 

Mental  receptivity  for  sound 

.     283 

Binaural  audition  ...... 

.     283 

THE  PHYSIOLOGICAL  CONDITIONS  OF  SENSATION 


APPENDIX   I 

The  action  of  light  on  the  retina    . 


299 


APPENDIX   II 

Derivations  of  scientific  terms 


302 


Index 


3i* 


LIST    OF    ILLUSTRATIONS 


FIG. 

PAGE 

I.   Cells  of  various  Forms       ......             7 

2.   Neuro-epithelial  Cell 

8 

3.   Section  of  Spinal  Cord 

10 

4.   Nerve  Fibres     ..... 

11 

5.   Multipolar  Nerve  Cells 

12 

6.   Pyramidal  Nerve  Cells 

12 

7.   Section  of  Spinal  Cord       ... 

16 

8.   Base  of  the  Brain       .... 

21 

9.  Diagram  of  Encephalon     . 

23 

10.  Diagram  of  Side  of  Brain  . 

25 

11.  Median  Aspect  of  Cerebral  Hemisphere 

27 

12.   Section  of  Skin           .... 

41 

13.  Grandry's  Corpuscles 

46 

14.  Wagner's  Corpuscle  .... 

47 

15.  Krause's  End-bulb     .... 

48 

16.   Large  End-bulb          .... 

48 

17.  Nerves  with  Pacinian  Corpuscles 

49 

18.   Pacinian  Corpuscle    .... 

50 

xvi  Physiology  of  the  Senses 

FIG. 

19.  Weber's  Compasses    . 

20.  Sieveking's  -Esthesiometer 

21.  Aristotle's  Experiment 

22.  Goldscheider's  Cold  and  Hot  Spots 

23.  Papilla  Foliata  . 

24.  Taste  bud 

25.  Section  of  Nasal  Cavities 

26.  Outer  side  of  Nares   . 

27.  Olfactory  Region  of  Rabbit 

28.  Olfactory  Cells  . 

29.  Section  of  Eyelid 

30.  Diagram  of  Eyeball   . 

31.  Section  of  Cornea 

32.  Section  of  Conjunctiva 

33.  Ciliary  Region  of  Eye 

34.  Vessels  of  Choroid  and  Iris 

35.  Retina 

36.  Retina 

37.  Rods  and  Cones 

38.  Ends  of  Rods  and  Cones 

39.  Pigment  Cells    . 

40.  Fibres  of  Lens  . 

41.  Diagram  of  Lens 

42.  Structure  of  Lens 

43.  Nerve  Fibres  in  Retina 

44.  Optic  Decussation 

45.  Reflection  of  Light    . 


PAGE 

54 
55 
61 

64 
7i 

73 
80 
82 
84 

85 
96 
98 

99 
100 
101 
102 
103 
104 

105 
106 
106 
107 
108 
109 
109 
no 

117 


List  of  Illustrations 


xvn 


FIG. 

PAGE 

46.   Refraction  of  Light 

Il8 

47.   Prism         ........ 

119 

48.  Lenses      ........ 

119 

49.   Biconvex  Lens  ....... 

120 

50.   Conjugate  Foci           ...... 

I20 

51.  Virtual  Focus 

I20 

52.  Formation  of  Image  ...... 

121 

53.   Effect  of  Absence  of  Lens  from  the  Eye 

122 

54.   Use  of  Lens  in  Formation  of  Image  in  the  Eye  . 

122 

55.   Spherical  Aberration           ..... 

123 

56.   Chromatic  Aberration         ..... 

I24 

57.  Achromatic  Lens        ...... 

125 

58.   Course  of  a  Ray  through  a  Dioptric  System 

126 

59.   Image  of  a  Point        .         .         .         . 

127 

60.   Schematic  Eye  ....... 

I30 

61.  Astigmatism      ....... 

132 

62.  Cylindrical  Lens  for  Astigmatism 

133 

63.  Adjustment  of  Eye  for  Distance 

134 

64.  Mechanism  of  Accommodation  .... 

135 

65.   Reflected  Images  in  Eye    ..... 

I36 

66.  Phakoscops       ........ 

136 

67.   Schemer's  Experiment        ...... 

137 

68.  Different  Forms  of  Eye      ...... 

139 

69.   Irradiation          ........ 

I4O 

70.   Formation  of  Purkinje's  Figures          .         .         .         . 

I42 

71.  Principle  of  the  Ophthalmoscope        . 

I44 

72.  The  Visual  Angle 

I46 

XV111 


Physiology  of  the  Senses 


FIG. 

73.  Small  Retinal  Images         ..... 

74.  The  Blind  Spot 

75.  Fusion  of  Retinal  Impressions    .... 

76.  Lambert's     Method     of     studying     Combinations  of 

Colours 

yj.  Diagram  to  illustrate  the  Young-Helmholtz  Theory 
of  Colour  Vision    ...... 

78.  Diagram    to    illustrate    Hering's    Theory   of  Colour 

Vision  ........ 

79.  The  Visual  Field 

80.  Diagram  of  Ocular  Muscles        .... 

81.  Section  through  the  Orbit  and  its  Contents 

82.  Binocular  Visual  Field       . 

83.  The  Horopter 

84.  Formation  of  Homonomous  Images    . 

85.  Formation  of  Heteronomous  Images  . 

86.  Truncated  Cone  seen  from  above 

87.  Wheatstone's  Stereoscope  ..... 

88.  Brewster's  Stereoscope       ..... 

89.  Telestereoscope  ...... 

90.  Causation  of  Luminosity    ..... 

91.  Estimation  of  Distance       .         .         . 

92.  Estimation  of  Space  ...... 

93.  Visual  Angle  in  Estimation  of  Size     . 

94.  Estimation  of  Size     ...... 

95.  Error  of  Judgment  in  Estimation  of  Size    . 

96.  Zollner's  Lines  ....... 


PAGE 

147 
150 
153 

156 

162 

166 
171 

172 

174 

176 

177 
178 

179 
181 
182 

183 
185 
187 

189 
189 

190 

191 

192 
192 


List  of  luustratiL 

ms 

XIX 

FIG.                                                                                                                                                               PAGE 

97.   Illusion  of  Vision       .... 

193 

98.   Perception  of  Solidity 

195 

99.  Auditory  Vesicle  of  Phialidium  . 

199 

100.   Right  Auricle    ... 

201 

1 01.   Diagram  of  the  Ear  .... 

203 

102.   Left  Tympanic  Membrane 

• 

205 

103.   Horizontal  Section  through  Ear 

206 

104.   Incus  and  Malleus  in  Tympanum 

207 

105.  Malleus 

209 

106.   Incus         ..... 

210 

107.   Stapes       ..... 

211 

108.  Leverage  Action  of  Malleus  and  Incu 

s 

213 

109.  Vibrating  Strings 

215 

110.  Wave-forms       .... 

220 

in.   Osseous  Labyrinth     . 

223 

112.  Formation  of  Semicircular  Canals 

224 

113.  Membranous  Labyrinth 

225 

114.   Section  of  Macula  Acustica 

226 

115.   Epithelium  of  Macula 

227 

116.   Otoconia  or  Otoliths 

227 

117.  Osseous  Cochlea 

228 

118.   Section  through  Coil  of  Cochlea 

229 

119.   Section  through  Cochlear  Duct  . 

231 

120.   RodsofCorti    .... 

232 

121.   Surface  View  of  Corti's  Organ   . 

233 

122.   Section  of  Corti's  Organ    . 

234 

123.  Diagram  of  Change  in  Breadth  of  the  I 

Sasilai 

•Men 

ibran 

2            239 

xx  Physiology  of  the  Senses 


PAGE 


124.  Double  Syren    ........  243 

125.  Pendular  Vibrational  Curves 249 

126.  Resonator          .         .         .         .         .         .         .         .  251 

127.  Konig's  Apparatus  for  studying  Vibration  of  Air  in 

Organ  Pipes          .......  254 


GENERAL   INTRODUCTION 

The  senses  are  called  into  play  when  the  condition  of  the 
body  has  been  affected  to  a  certain  degree  by  external  or 
internal  agencies.  A  flash  of  light,  a  piercing  sound,  a 
gentle  touch,  may  so  act  upon  the  bodily  organism  as  to  be 
followed  by  a  sensation  or  mental  state,  by  the  conscious- 
ness of  an  alteration  that  has  taken  place  in  the  body  or  in 
its  environment.  Sensitiveness  is  a  property  of  all  animals, 
and  possibly  of  not  a  few  plants.  Some  animals,  indeed, 
are  so  low  in  the  scale  of  organisation  as  to  have  no  special 
parts  set  aside  for  the  reception  of  sensory  impressions,  but 
every  part  of  their  body  seems  alike  fitted  to  recognise  varia- 
tions in  its  surroundings.  As  soon,  however,  as  we  pass  to 
the  higher  grades  of  animal  life  we  find  certain  parts  or 
organs  of  sense  whose  duty  is  to  keep  the  body  in  touch 
with  its  surroundings,  and  a  nervous  system  which  receives 
impressions  and  ensures  the  co-operation  of  all  the  individual 
elements  of  the  body  one  with  another. 

In  order  that  sensations  may  be  felt,  we  are  provided 
with  a  central  nervous  system,  or  sensorium,  from  which 
nerve  fibres  pass  outwards  to  all  parts  of  the  body,  and  at 
the  ends  of  the  nerve  fibres  certain  structures  or  terminal 
organs  may  be  found,  which  are  so  formed  as  to  be  capable 
of  responding  to  some  special  variety  of  impression.  Thus 
the  terminal  organ  of  the  nerve  of  vision  is  insensitive  to 


2  Physiology  of  the  Senses 

the  vibrations  which,  by  acting  upon  the  ear,  originate 
changes  leading  to  the  sensation  of  sound.  But,  as  will  be 
shown  in  greater  detail  hereafter,  this  receptivity  is  largely 
conditioned  by  the  special  function  of  each  sensory  nerve 
centre.  For  the  sensorium  does  not  act  as  a  whole,  but  is 
differentiated  so  that  one  part  is  devoted  to  one  sense, 
another  to  another ;  and  when  the  nerves  which  lead  to 
these  nerve  centres  have  been  stimulated,  it  matters  not 
what  the  nature  of  the  stimulus  to  the  nerve  has  been,  the 
sensation  experienced  is  always  for  each  centre  of  one  and 
the  same  kind.  Thus  the  optical  centre  always  gives  rise 
to  the  sensation  of  seeing  something,  the  auditory  centre  to 
that  of  hearing,  the  olfactory  centre  to  sensations  of  smell, 
the  gustatory  centre  to  those  of  taste,  and  the  tactile  centre 
to  touch.  But,  over  and  above  these  special  forms  of 
sensation,  there  are  many  vague  or  general  sensations,  such 
as  those  of  heat  or  cold,  of  pain  or  fatigue,  of  pressure, 
resistance,  and  the  like,  which  may  seem  to  be  felt  in 
almost  every  part  of  the  body  ;  and  although  each  of- these 
has  in  all  probability  its  special  nerve  centre,  yet  no  special 
terminal  organ  seems  to  be  necessary. 

Special  terminal  organs,  then,  are  developed  for  the 
senses  of  sight,  hearing,  smell,  taste,  and  touch :  their  structure 
will  be  described  when  we  consider  these  senses  separately. 

While  we  may  readily  distinguish  these  organs  from  one 
another  by  examination,  either  with  the  naked  eye  or  the 
microscope,  it  is  quite  otherwise  when  we  come  to  study 
the  nerve  fibres  or  nerve  centres.  So  far  as  we  can  as  yet 
determine,  the  nerve  fibres  which  transmit  the  various 
sensory  impressions  are  all  of  exactly  the  same  composition 
and  structure  ;  and  though  in  recent  times  it  has  been 
found  possible  to  localise  with  considerable  accuracy  the 
centres  which  are  related  to  special  sensations,  still  it  has 
not    been    possible    to   fix    upon    the    exact    microscopical 


General  Introduction  3 

elements  concerned  ;  in  other  words,  physiologists  cannot 
define  the  particular  structure  which  alone  is  concerned  in  a 
given  special  sensation.  We  have  no  means  of  observing 
directly  the  minute  molecular  changes  which  go  on  in  nervous 
substance  ;  we  know  only  that  this  substance  is  very  complex, 
and  that  during  life  it  undergoes  continual  change,  and  is 
being  constantly  built  up  and  broken  down  ;  but  neither  the 
microscope  nor  chemical  analysis  has  hitherto  enabled  us  to 
determine  why  one  centre  should  respond  to  one  form  of 
physical  change,  and  another  to  another ;  or  why,  when  one 
part  is  stimulated,  we  have  one  kind  of  sensation,  and  when 
another  part  acts  we  have  a  different  kind. 

A  brief  consideration  of  the  composition  and  structure  of 
nerve  fibres  and  of  nerve  centres  will  enable  us,  however, 
to  understand  better  the  mechanism  required  for  the  trans- 
mission and  recognition  of  a  sensory  impression. 

Nerve  matter  consists  mainly  of  a  variety  of  the  sub- 
stance called  protoplasm,  which  is  composed  of  a  network 
of  exceedingly  fine  fibres,  the  meshes  of  which  are  filled  up 
with  a  fluid  or  semi-fluid  substance.  The  exact  chemical 
nature  of  protoplasm  cannot  be  stated,  for,  in  the  first  place, 
it  is  constantly  varying  during  life  by  taking  up  nutrient 
matter  of  different  kinds,  and  by  throwing  off  certain  waste 
substances,  the  product  of  vital  action  ;  and  in  the  second 
place,  whenever  we  try  to  subject  it  to  chemical  analysis,  it 
dies  and  is  broken  up  into  simpler  chemical  compounds. 
The  most  important  chemical  elements  found  in  protoplasm 
are  Carbon,  Oxygen,  Hydrogen,  Nitrogen,  Sulphur,  and 
Phosphorus,  and  they  are  combined  in  such  quantities  and 
proportions  as  to  form  molecules  of  a  highly  complex  nature. 
Now  the  more  complex  a  chemical  compound  is,  the  more 
unstable  it  is ;  or,  in  other  words,  the  more  easily  may  it 
be  broken  up,  and  resolved  into  simpler  substances  ;  and 
hence  we  have  in  nervous  tissues,  which  are  largely  com- 


4  Physiology  of  tJie  Senses 

posed  of  protoplasm,  a  material  which  may  be  very  readily 
changed  when  acted  upon  by  external  forces. 

That  a  change  does  take  place  in  nerve  matter,  when  in 
action,  has  been  inferred,  although  we  cannot  tell  what  the 
exact  chemical  constitution  of  nervous  matter  is,  nor  how 
it  is  changed.  We  know  that  for  the  efficient  working  of  the 
nervous  system  there  must  be  a  full  and  unrestricted  blood 
supply,  bringing  fresh  nutrient  matter  to  make  up  for  waste, 
and  oxygen,  to  promote  chemical  changes.  The  blood, 
again,  must  be  free  from  impurities,  or  nerve  action  will  be 
disordered.  Surrounding  nerve  fibres  we  find  a  system  of 
fine  spaces  or  channels  into  which  waste  products  of  nerve 
action  are  poured,  so  as  to  secure  their  ready  removal. 
When  a  nerve  is  acting  we  can  also  detect  electrical 
changes  corresponding  in  all  probability  to  chemical  trans- 
formations of  nerve  substance,  but  it  must  be  admitted  that 
no  proof  has  yet  been  given  of  chemical  changes  in  a  n^rve. 

Nerves. — When  a  nerve  has  been  kept  in  action  for  some 
time  it  apparently  becomes  fatigued — that  is  to  say,  the 
irritation  of  the  nerve  ceases  to  be  followed  by  the  usual 
result.  Thus,  if  we  irritate  a  nerve  passing  to  a  muscle, 
the  muscle  at  first  responds  by  contracting,  but  by 
and  by  the  stimulations  of  the  nerve  fail  to  call  forth 
contraction.  We  then  say  the  nerve  is  fatigued,  and  we 
may  suppose  that  its  vital  activity  is  diminished  from  lack 
of  time  to  build  up  its  wasted  substance,  or  from  the  ac- 
cumulation of  waste  products  which  prevent  free  action. 
Of  late,  however,  physiologists  are  gradually  coming  to  the 
opinion  that  there  is  no  direct  evidence  of  fatigue  in  the 
nerve  itself,  and  that  the  phenomena  on  which  fatigue 
depends  really  occur  in  the  apparatus  or  structure  at  the 
end  of  the  nerve.  From  this  point  of  view,  nerve  fibres 
may  be  regarded  as  not  subject  to  much  tear  and  wear, 
and  they  may  act  more  like  metallic  conductors  conveying 


General  Introduction  5 

currents  of  electricity,  in  which  the  current  does  not  produce 
what  are  usually  called  chemical  phenomena. 

Nature  of  Nerve  Current. — Structures  known  as  nerve 
cells   maintain   the   nutrition   of  nerve  fibres.       If  a  fibre 
is  cut  off  from  the  cell  with  which  it  is  connected  it  soon 
degenerates,  and  can  no  longer  transmit  a  nerve  current.   But 
in  a  healthy  nerve  fibre  a  change  known  as  a  "  nerve  current  " 
passes  along  it  in  both  directions  of  its  length  from  the  point 
of  stimulation.      This  change  may  be  of  a  chemical  kind, 
although,  as  already  pointed  out,  there  is  no  proof  of  this,  and 
certain  facts  point  the  other  way.     We  may  imagine,  on  the 
chemical  hypothesis,  the  fine  nerve  fibre  as  containing  very 
complex  and  unstable  molecules,  which  are  readily  broken 
up  when  acted  upon  by  some  external  force.      And  just  as 
when  a  match  is  set  to  one  end  of  a   train  of  gunpowder, 
the  chemical  change  in  the  first  granules  of  powder  liberates 
energy,  which  gives  rise  to  action  in  adjoining  granules  with 
disintegration    of    their    substance    and    the    formation    of 
simpler  compounds,  so  in  nerve  the  change  in  one  part  or 
molecule  may  give  rise  to  changes  in  adjoining  molecules, 
and  a  so-called  current  will  pass  along  the  fibre.      The  fact 
that  one  current  may  follow   another  with   great   rapidity 
shows  that  the  nerve  substance  is  altered  only  in  part  and 
is  quickly  regenerated ;   but,   on    the   other   hand,  the   too 
frequent  or  prolonged  application  of  a  stimulus  is  followed 
by  diminished   power   of   conductivity   by   a   nerve,    or  of 
receptivity  in  the  nerve  centres.      It  was  at  one  time   sup- 
posed that  the  nerve  current  might  be  a  purely  electrical 
change,  and  that  it  travelled  with  the  lightning  velocity  of 
the     electric     current.      And    no    doubt    in     our    ordinary 
experience   this   seems   to   be   the    case.      If  the   skin    be 
touched  with  a  red-hot   iron  wire,  we  seem  at  the   same 
instant  to  feel  the  heat  and  pain.      But  by  means  of  ap- 
paratus for   registering   minute   intervals   of  time,  and   by 


6  Physiology  of  the  Senses 

stimulating  a  nerve  in  different  parts  of  its  length,  we  have 
ascertained  that  the  rate  of  the  nerve  current  is  much 
slower  than  it  would  be  were  it  purely  electric  ;  and  while 
there  may  be  electric  disturbance  due  to  chemical  change 
of  the  substance  of  the  nerve  fibre,  that  disturbance  is 
probably  only  a  minor  part  of  the  phenomenon.  The 
electric  flash  passes  at  the  rate  of  thousands  of  miles,  the 
nerve  current  never  faster  than  200  feet,  per  second.1 

This  rate  of  transmission  of  a  nerve  impulse  must  how- 
ever be  carefully  distinguished  from  the  time  occupied  by 
nerve  centres  in  undergoing  those  changes  which  may  or 
may  not  lead  to  consciousness  or  the  perception  of  the 
sensation.  Thus  if  it  be  arranged  that  a  person  shall 
make  a  signal  as  quickly  as  possible  after  seeing  a  flash 
of  light,  it  is  found  that  the  time  which  elapses  between 
the  two  events  will  be  greater  than  would  be  required  for 
the  sensory  impulse  to  pass  to  the  sensory  centre^  and 
thence  by  efferent  nerves  to  the  muscles  of  the  limb  by 
which  the  movement  is  effected.  There  is  time  required 
for  the  supervention  of  the  conscious  state,  and  for  the 
generation  of  the  volition  which  leads  to  the  movement. 
This  interval  has  been  called  the  psycho-physical  time, 
because  we  have  here  to  do  not  merely  with  changes  in 
nerve  matter,  but  also  with  mental  conditions  and  acts. 
The  psycho-physical  time  varies  considerably  under  different 
circumstances.  Thus,  for  example,  less  time  will  be  required 
if  the  observer  has  merely  to  make  a  prearranged  signal 
that  he  has  become  conscious  of  some  given  sensory  stimulus 
— -the  so-called  perception  time — than  if  he  be  asked  to 
decide  between  two  sensations,  as  of  a  low  and  high  sound, 

1  Recently  it  has  been  suggested  that  the  nervous  impulse  is  elec- 
trical, and  that  its  velocity  is  slow  compared  with  the  velocity  of  elec- 
tricity, because  great  delay  occurs  at  certain  points  along  the  fibre, 
known  as  the  nodes  of  Ranvier.  No  positive  proof  has  yet  been 
offered  of  this  somewhat  fascinating  theory. 


General  Introduction 


or  a  bright  or  dull  colour.  This  latter  task  requires  nearly 
half  a  second  of  time.  Even  longer  time  is  involved  when 
the  observer  has  to  make  a  choice  as  to  which  of  two 
stimuli  he  shall  signal — somewhat  more  than  half  a  second 
being  usually  required. 

On  the  other  hand,  it  is  possible  that  a  stimulus  to  a 
sensory  nerve  may  give  rise  to  movement  quite  indepen- 
dently of  consciousness  and  volition.  In  this  case  the 
sensory  impulse  affects  certain  nerve  centres,  either  in  the 
spinal  cord  or  the  base  of  the  brain,  which  are  able  so  to 
respond  as  to  cause  an  efferent  current  to  bring  about  some 


Fig.  i. — Various  forms  of  cells,  a,  cylindrical  or  columnar ;  b,  caudate  or 
tailed  ;  c,  fusiform  or  spindle-shaped  ;  d,  ciliated,  having  fine  filaments  pro- 
iecting  from  their  free  surface  ;  e,  stellate  or  branched. 

muscular  action.  In  this  case  the  time  occupied  in  the 
nerve  centre  is  less  than  when  volition  is  involved,  but  is, 
however,  greater  than  would  be  required  for  the  simple 
passage  of  the  nerve  current  along  a  nerve.  It  amounts  to 
about  .05  of  a  second. 

Origin  of  Nervous  System. — We  have  said  that  the 
nerves  are  largely  composed  of  protoplasm.  But  this 
substance  exists  in  all  parts  of  the  body,  at  least  in 
early  life.  When  we  examine  microscopically  the  tissues 
of  the  body  during  the  earliest  periods  of  its  existence, 
we  find  that  it  is  composed  of  minute  vital  elements  to 
which   the   name   of  cells   or   corpuscles  has  been  given. 


8  Physiology  of  the  Senses 

These  cells  are  composed  of  protoplasm,  and  usually 
contain  an  exceedingly  minute  body,  called  the  nucleus, 
whose  composition  is  in  certain  respects  different  from  that 
of  protoplasm,  and  the  cells  may,  moreover,  be  surrounded 
by  a  cell  wall  of  less  actively  vital  matter.  At  first  the 
various  cells  of  the  body  closely  resemble  one  another,  but  as 
growth  advances  they  become  differentiated  in  form  (Fig.  i) 
and  structure  in  order  to  perform  special  functions,  some 
cells  going  to  build  up  the  skin,  some  the  muscles,  some 
the  nervous  tissues  and  the  like.  In  low  forms  of  animal 
life,  however,  these  cells  are  often  not  so  highly  differ- 
entiated as  in  man.  Thus  in  the  sea-anemone  {Actinia), 
among  the  cells  which  go  to  form  the  outer  covering 
or  skin,  we  find  certain  cells  from  the  free  surface  of  which 
a  hair -like  filament  projects,  while  from  their  attached 
border  a  number  of  processes  pass  inwards  and  join  with 

like  processes 
from  other  similar 
cells.  These  hair 
cells  form  rudi- 
mentary sense 
organs  (Fig.  2). 

Fig.  2. — Neuro-epithelial  cell  from  the  upper  nerve  ring  ,  .       , 

of  Carmina  hastata.     c,  sense  hair  passing  to  the  ' 

surface  ;  the  two  long  thin  processes  join  a  ring  of    network       formed 
nerve  fibres  containing  ganglion  cells.     (Hertwig.)         ,         ,-,  r 

&  &    &  by  the   union    of 

the  processes  just  mentioned  may  be  found  cells  which  seem 
to  have  sunk  inwards  from  the  surface  showing  like  processes, 
and  regarded  by  Balfour 1  as  an  elementary  sensory  nervous 
apparatus.  In  general,  it  may  be  said  that  a  study  of  the 
facts  of  development  shows  us  that  nerve  cells  appear  at 
first  upon  the  surface  of  the  body,  but  that  during  the 
growth  of  the  organism  the  cells  become  shut  off  from  the 
surface;  and  in  order  to  maintain  their  connection  with  the 

1   F.  M.  Balfour,  Comparative  Embryology,  vol.  ii.  p.  332. 


General  Introduction  9 

periphery,  long  processes  called  nerve  fibres  pass  from 
the  cells  thus  deeply  embedded  to  the  surface. 

Nerve  cells  may  occur  singly,  or  more  commonly  they 
are  found  gathered  together  in  groups  called  ganglia,  the 
individual  cells  being  known  as  ganglionic  nerve  cells.  These 
ganglionic  cells  are  more  or  less  closely  connected  with  one 
another  by  means  of  nerve  fibres,  and  thus  community  of 
action  is  established. 

In  insects,  for  example,  we  find  two  rows  of  ganglia,  the 
cells  of  which  are  united  by  nerve  fibres  both  longitudinally 
and  transversely.  Sensory  impressions  pass  by  nerve  fibres 
to  these  ganglia,  and  again,  by  other  fibres  passing  out 
from  these  ganglia  and  ending  in  muscular  tissue  the  move- 
ments of  the  body  are  regulated.  In  insects,  too,  it  may  be 
noted  that  the  ganglia  connected  with  organs  of  special  sense, 
such  as  the  eye  or  ear,  are  larger  than  the  others.  A  further 
development  of  the  nervous  system  arises  through  the 
fusion  of  ganglia  with  each  other,  so  that  the  brain  and 
spinal  cord  of  vertebrate  animals  may  be  regarded  as  a  vast 
number  of  ganglionic  cells  and  nerve  fibres  bound  into 
one  consistent  whole  by  a  fine  network  of  a  connective 
tissue,  and  by  an  interlacing  of  nerve  fibres. 

The  nerve  fibres  connected  with  the  brain  and  spinal  cord 
may  be  divided,  according  to  their  function,  into  two  sets — 
those  which  transmit  sensory  impressions  inwards,  the 
afferent  nerves,  and  those  which  have  to  do  with  the 
regulation  of  such  changes  in  the  body  as  lead  to  motion  or 
secretion,  and  known  as  efferent  nerves.  Thus  the  sensa- 
tion of  pain,  as,  for  example,  toothache,  originates  from 
stimulation  of  a  sensory  or  afferent  nerve  ;  and  the  move- 
ments involved,  say,  in  swallowing,  from  stimulation  of 
efferent  nerves  passing  outwards  from  the  brain  or  cord. 

Structure  of  Nerves  and  Nerve  Cells. — The  progress 
of  research  tends  to   show  that  fibres  of  varying  function 


IO 


Physiology  of  the  Senses 


always  occupy  a  similar  relative  position  in  the  central 
nervous  system.  As  long  ago  as  1822,  Majendie  showed  that 
the  afferent  or  sensory  fibres  always  pass  into  the  spinal 
cord  by  what  is  known  as  the  posterior  root  of  a  spinal 
nerve,  while  efferent  or  motor  fibres  emerge  from  its  anterior 
aspect.  See  Fig.  3.  But  it  -has  been  found  a  matter  of 
the  greatest  difficulty  to  determine  accurately  the  course 
of  fibres  in  the  cord  itself.  When  we  look  with  the 
naked  eye  at  a  cross  section  of  the  spinal  cord,  we  can  see 
at  a  glance  that  it  is  made  up  apparently  of  two  kinds  of 
material,  the  outer  part  being  whiter  than  the  inner,  which  is 


r  *       1 

Fig.  3. — Portion  of  the  spinal  cord  from  the  region  of  the  neck,  with  roots  of  the 
nerves  (slightly  enlarged),  i,  i,  The  anterior  median  fissure  ;  2,  the  posterior 
median  fissure  ;  3,  the  anterior  lateral  groove,  from  which  the  anterior  roots 
of  the  nerves  are  seen  emerging  ;  4,  posterior  lateral  groove  where  the  pos- 
terior nerve  roots  enter  the  cord  ;  5,  anterior  roots,  to  the  right  passing  the 
ganglion  ;  5',  anterior  root  cut  across  ;  6,  posterior  root  with  ganglion  at  6'  *. 
7,  the  nerve  made  up  of  anterior  and  posterior  fibres ;  7',  the  first  branches 
from  the  compound  nerves.     (Allen  Thomson.) 

of  a  gray  colour.  This  whiteness  is  due  to  the  fact  that  the 
protoplasmic  substance  of  the  nerve  fibre,  the  part  which 
conveys  the  nerve  current,  the  so-called  axis  cylinder  of  the 
nerve,  is,  in  the  greater  part  of  its  length,  surrounded  by  a 
sheath  of  fatty  material,  known  as  the  white  substance  of 
Schwann  (Fig.  4),  which  in  bulk  gives  a  creamy  white 
appearance  to  a  group  of  nerve  fibres.  This,  in  turn,  is 
enclosed  by  a  thin  transparent  covering  known  as  Schwann's 
sheath,  or  the  primitive  sheath.  But  in  the  central  parts  of 
the  cord  the  white  substance  is  to  a  large  extent  absent,  and 
we  here  find  among  the  fibres  great  numbers  of  ganglionic 


General  Introduction 


ii 


nerve  cells.  These  cells  vary  much  in  shape,  but  are  mostly 
of  the  form  called  multipolar,  on  account  of  the  large 
number  of  poles  or  nerve  fibres  which  spring  from  them 
(Fig.  5),  while  others,  and  more  especially  the  cells  in  the 
posterior  part  of  the  gray  matter,  are  often  spindle-shaped 
or  pyramidal  (Fig.  6).  These  cells  are  in  direct  connection, 
for  the  most  part,  with  efferent  motor  nerves  ;  and  if  they 
are  destroyed  by  disease  or  otherwise,  the  nerve  fibres  with 
which  they  are  connected  quickly  degenerate,  and  the  parts 
supplied  by  them  are  paralysed.  These 
are  the  cells  which  may  be  roused  to 
action  by  the  sensory  nerves  quite 
apart  from  any  conscious  sensation.  If 
the  foot  of  a  person  in  profound  sleep 
be  lightly  tickled,  it  will  be  drawn 
away  without  the  sleeper  being  dis- 
turbed. If  the  middle  or  upper  parts 
of  the  spinal  cord  be  destroyed  with- 
out injury  to  the  lower  part  of  the 
cord,  while  sensory  impressions  can 
pass  to  this  lower  part,  and  can  set 
up  changes  in  the  nerve  cells  which 
lead  to  the  movement  of  the  lower 
part  of  the  body  or  legs,  these  move- 
ments are  performed  unconsciously, 
and  therefore  cannot  be  controlled  or 
restrained  by  an  act  of  will,  since  the 
impression  is  not  transmitted  to  the 
brain.  Man  is  only  conscious  when 
certain  parts  of  his  brain  have  been 
affected.  Unless  sensory  impressions  are  transmitted  to 
these  parts,  or  unless  these  parts  have  been  called  into 
action  by  some  variation  in  their  chemical  composition, 
there  will   be   no   consciousness.       If   these   parts   are   ill- 


Fig.  4. — Nerve  fibres.  JB, 
The  axis  -  cylinder  sur- 
rounded by  the  white 
substance  of  Schwann, 
which  is  interrupted  at 
A,  a  node  of  Ranvier, 
and  contains  a  nucleus 
at  C.  The  external  line 
represents  the  primitive 
sheath  or  neurilemma. 


12 


Physiology  of  the  Senses 


developed  and  ill -nourished,  sensation  will  be  feeble  or 
perverted ;  and  if  they  are  destroyed,  the  possibility  of 
consciousness  will  be  permanently  lost. 


Fig.  5. — Multipolar  nerve  cells  in  the  anterior  part  of  the  gray  matter  of  the 
spinal  cord,  ar,  anterior  roots  of  emergent  nerve  fibres  coming  from  the 
nerve  cells,  gc  ',  nf,  nerve  fibres  cut  across. 


Paths  of  Nervous  Impulses 
1 .  The  Spinal  Cord. — When  we  seek  the  exact  paths,  how- 


Fig.  6.— Pyramidal  nerve  cells  found  principally  in  the  brain. 


General  Introduction  1 3 

ever,  along  which  sensory  impulses  pass  up  the  cord  to  the 
brain,  we  are  met  by  many  difficulties.  We  can  only  infer 
that  an  animal  feels  some  sensation  ;  we  cannot  enter  into  its 
consciousness  of  it.  When  the  foot  of  an  animal  is  pinched 
we  believe  that  it  feels  pain  because  of  some  movement  it 
makes,  or  some  sound  it  utters,  and  because  we  know  that 
a  similar  pinch  to  our  own  feet  would  cause  a  sensation  of 
pain  in  us.  But  if,  by  careful  and  gradual  operation,  the 
greater  part  of  the  brain  has  been  removed  and  the 
animal  has  survived,  we  find  that  the  application  of  the 
stimulus  may  still  educe  movements  or  cries,  while  we 
cannot  suppose  the  animal  to  be  conscious  of  what  it  does. 
Another  difficulty  in  the  determination  of  the  sensory  path 
is  that  of  isolating  or  destroying  a  certain  part  of  the  cord 
without  injury  to  other  parts,  and  without  setting  up  irrita- 
tion or  shock  which  may  lead  to  erroneous  inferences.  It 
is  impossible  to  reach  the  deeper  parts  of  the  cord  without 
injuring  the  more  superficial,  and  the  individual  fibres  are 
so  small  that  it  is  very  much  a  matter  of  guess-work  whether 
we  have  cut  the  parts  we  wish  or  not.  We  know  that 
sensory  fibres  enter  at  the  posterior  part  of  the  cord,  that 
some  of  these  fibres  pass  directly  into  the  gray,  some  into 
the  white,  matter ;  but  hitherto  it  has  not  been  possible 
to  trace  these  fibres  to  any  extent,  on  account  of  their 
bending  away  from  the  plane  of  section.  It  has  been 
observed  that  at  different  stages  of  development  certain 
strands  of  fibres  are  superposed,  as  it  were,  on  others  ;  and 
by  examining  sections  of  cords  of  animals  at  different  ages 
the  connections  of  special  tracts  have  been  traced. 

Another  method  of  study  which  has  afforded  valuable 
results  is  based  upon  the  observation  that  when  nerve 
fibres  have  been  cut  off  from  the  nerve  cells  with  which 
they  are  connected,  the  fibres  quickly  degenerate ;  and 
thus   it   has  been  found   possible  to  trace  the  line   of  de- 


14  Physiology  of  the  Senses 

generation  for  some  distance.  Similarly,  in  cases  of  loss 
of  sensation  in  disease,  it  may  be  possible  to  discover,  by 
post-mortem  examination,  the  part  which  has  suffered ; 
but  it  will  readily  be  seen  that  this,  and  the  above- 
mentioned  methods  of  research,  can  only  afford  rough  and 
inaccurate  results.  One  interesting  fact  we  can  con- 
clusively settle  from  cases  of  disease  in  the  human  being 
is,  that  different  kinds  of  sensations  travel  by  different 
paths  in  the  cord.  A  lesion  which  may  cut  off  the  pos- 
sibility of  feeling  pain  in  a  given  part  of  the  body,  may 
leave  it  still  susceptible  to  sensations  of  heat  and  cold  ; 
or  the  sensation  of  touch  may  be  present  while  the  sensa- 
tion of  pain  cannot  be  aroused.  From  this  we  see  that 
nerve  impulses  giving  rise  to  sensations  of  touch,  of  pain, 
of  temperature,  of  the  muscular  sense,  must  pass  upwards 
to  the  sensorium  by  different  paths,  one  of  which  may  be 
cut  off  while  the  others  remain.  We  may  also  learn 
from  such  cases  that  the  sensory  fibres,  after  passing  up 
the  cord,  terminate  in  the  opposite  side  of  the  brain  from 
that  in  which  we  seem  to  have  the  sensation. 

Where  the  sensory  fibres  cross  from  one  side  to  the 
other  is  not  known.  The  experiments  of  the  older  physio- 
logists, and  more  especially  those  of  the  French  observer, 
Brown -Sequard,  seemed  to  show  that  the  sensory  fibres 
cross  to  the  other  side  almost  immediately  after  their 
entrance  into  the  cord  ;  but  later  workers  in  this  field  of 
research  maintain  that  the  majority  of  the  sensory  fibres 
do  not  cross  at  once,  but  pass  up  almost  to  the  base  of  the 
brain  before  they  change  sides.  In  some  parts  of  the  cord, 
however,  the  fibres  do  cross  from  the  right  to  the  left  side, 
and  vice  versa,  or  decussate,  as  it  is  called  ;  so  that  sensory 
fibres  from  the  right  side  of  the  body  pass  to  the  left  side 
of  the  brain,  and  from  the  left  side  of  the  body  to  the  right 
side  of  the  brain.      It  is  probable  that  they  do  not  extend 


General  Introduction  15 

continuously,  however,  as  single  threads,  from  the  peri- 
phery to  the  sensorium.  We  have  seen  that  the  stimulation 
of  a  sensory  nerve,  say  in  the  right  foot,  may  give  rise  to 
changes  in  the  lower  part  of  the  cord,  and  hence  to  involuntary 
movements  of  which  we  are  totally  unconscious  ;  or  it  may 
cause  a  sensation  by  stimulation  of  the  brain.  Now  we 
do  not  find  nerve  fibres  branching  except  at  their  endings. 
Hence  we  are  led  to  conjecture  that  the  majority  of  the  sensory 
fibres  pass  immediately  into  the  gray  matter  of  the  cord 
and  there  become  connected  with  nerve  cells.  From  these 
some  fibres  may  pass  to  the  cells  in  the  cord  connected  with 
efferent  nerves,  while  other  fibres  pass  upwards  to  the  brain. 
To  give  a  slightly  more  definite  idea  of  the  paths  pur- 
sued by  the  different  sensory  fibres,  we  may  refer  to  Fig. 
7,  in  which  we  have  a  diagrammatic  representation  of  a 
transverse  section  of  the  spinal  cord  divided  into  tracts  or 
areas,  which  are  to  be  understood  as  indicating  bundles  or 
columns  of  fibres  running  side  by  side  and  communicating 
freely  with  one  another,  but  each  containing,  in  the  main, 
fibres  of  special  origin  and  function.  Thus,  for  example, 
the  nerve  fibres  which  convey  painful  impressions  appar- 
ently pass  into  the  gray  matter  of  the  cord,  for  if  the  gray 
matter  be  completely  divided  at  any  given  level  of  the 
cord,  there  will  no  longer  be  a  sensation  of  pain  when  the 
parts  are  injured  which  send  nerve  fibres  to  the  cord  below 
the  level  of  section.  From  the  gray  matter  fibres  prob- 
ably pass  outward  and  upward  in  the  anterior  root  zone 
(ar,  ar',  Fig.  7).  Suppose  the  gray  matter  were  divided  close 
above  the  region  where  sensory  fibres  from  the  legs  pass 
into  the  cord.  Then  we  might  lacerate  the  foot,  and 
though  we  might  feel  that  it  was  being  touched,  we  would 
have  no  sensation  of  pain  from  the  operation.  We  distin- 
guish, therefore,  between  a?ialgesia,  or  that  condition  in 
which  painful  sensations  cannot  be  excited,  and  anesthesias 


i6 


Physiology  of  the  Senses 


or  the  state  in  which  we  are  insensitive  to  tactile  sensa- 
tions. It  will  readily  be  understood  that  analgesia  of  any 
part  of  the  body  might  lead  to  disastrous  consequences. 
Thus  among  paralytics  we  find  patients  who  feel  no  pain 
in,  and  are  unable  to  move,  the  lower  limbs.  They  will 
allow  some  part,  such  as  the  heel,  to  remain  motionless  on 


ah        ctr 


dc 


&-' 


Fig.  7. — Transverse  section  of  human  spinal  cord,  ah,  ah',  anterior  horns  of 
gray  matter  ;  ph,  ph' ,  posterior  horns  of  gray  matter  ;  ar,  ar1 ,  anterior  root 
zones  ;  pr,pr ',  posterior  root  zones  ;  P,  P',  pyramidal  fibres  of  lateral  columns 
(mainly  motor  in  function) ;  T,  columns  of  Tiirck  (motor  in  function) ;  G, 
columns  of  Goll ;  dc,  dc' ,  direct  cerebellar  tract ;  c,  anterior  commissure  ; 
below  c,  central  canal  of  cord  lined  with  columnar  epithelium.  (Ross  and 
Young.) 

a  couch  so  long  that  the  circulation  of  blood  in  it  ceases, 
and  its  vitality  may  be  seriously  impaired.  Similarly 
where  the  front  of  the  eyeball  has  become  insensitive  to 
pain,  the  presence  of  small  foreign  bodies  in  the  eye  being 
no  longer  felt,  such  bodies  accumulate  in  the  eye,  interfere 
with  its  well-being,  and  give  rise  to  ulceration  and  de- 
struction of  the  ball.  To  the  healthy  body  pain  is  nature's 
indicator  of  danger  ;  the  burnt  child  dreads  the  fire. 


General  Introduction  17 

Tactile  impressions  in  man  pass  upward,  for  the  most 
part,  in  those  columns  of  the  cord  which  lie  between  the 
posterior  roots  of  the  spinal  nerves.  In  this  part,  besides 
the  paths  for  the  stimuli  which  give  rise  to  the  sense  of 
touch,  we  have  probably  also  those  which  excite  the  sensa- 
tions of  heat  and  cold,  of  pressure  and  resistance,  and  of 
tickling.  That  this  is  so  is  most  distinctly  shown  by  the 
study  of  changes  in  the  cord  during  the  progress  of  the 
disease  known  as  locomotor  ataxia — a  disease,  one  promi- 
nent symptom  of  which  is  disorder  of  the  power  of  walking. 
Patients  subject  to  this  disease  usually  suffer,  in  the  earlier 
stages,  from  severe  pains  shooting  apparently  into  the  legs, 
and  due  to  inflammatory  changes  in  the  posterior  horns  of 
the  gray  matter.  Then  the  areas  immediately  adjoining 
these  (fir,  fir,'  Fig.  7)  become  diseased,  and  the  muscular 
sense  is  impaired,  so  that  there  is  not  the  accustomed 
guide  to  the  muscles  as  to  the  amount  of  force  required  for 
movement,  and  the  patient  tends  to  lift  his  feet  too  high 
and  to  set  them  down  with  a  stamp.  He  is  not  able  to 
judge  accurately  as  to  the  weight  of  his  limbs,  nor  of 
heavy  masses  attached  to  them.  Then  the  delicacy  of  his 
sense  of  touch  becoming  impaired,  he  has  the  feeling,  even 
when  walking  on  rough  ground,  as  if  he  were  treading  on 
velvet.  No  longer  receiving  the  wonted  guiding  im- 
pressions from  his  feet,  he  must  watch  with  his  eyes  his 
movements  in  walking,  directing  his  steps  by  his  sense  of 
sight,  andHf  he  shuts  his  eyes  he  staggers  and  falls.  His 
muscles  act  spasmodically,  independently  of  each  other, 
without  due  co-ordination.  At  first  the  motor  power  re- 
mains, but  eventually  it  too  may  become  involved,  and  the 
patient  is  paralysed  for  motion  as  well  as  sensation. 

In  some  animals,  such  as  rabbits,  it  has  been  supposed 
that  the  tract  for  tactile  sensations  is  in  the  lateral  columns  ; 
but  all  experiments  on  the  sensory  tracts  are  very  apt  to 

c 


1 8  Physiology  of  the  Senses 

be  deceptive  from  the  difficulty  of  interpreting  the  resulting 
phenomena. 

As  the  sensory  tracts  pass  upward  in  the  spinal  cord 
they  are  somewhat  modified  in  size  and  in  relative  position, 
owing  to  intercommunication  and  the  entrance  of  fresh 
fibres,  but  on  the  whole  the  strands  preserve  the  same 
general  relationship.  But  just  as  the  cord  enters  the 
cavity  of  the  skull  it  enlarges,  to  form  a  portion  about 
an  inch  and  a  quarter  long,  known  as  the  bulb  or  ?nedulla 
oblongata.  Here  the  arrangement  of  the  white  and  gray 
matter  is  much  modified,  and  mixed  with  the  fibres  con- 
ducting nerve  impulses  to  and  from  the  brain  we  find 
several  ganglionic  centres  which  are  of  vital  importance. 
Here,  for  example,  we  find  centres  which  preside  over  the 
great  functions  of  respiration  and  the  circulation  of  the 
blood,  besides  such  as  regulate  the  acts  of  mastication 
and  of  swallowing,  vocal  utterance,  the  secretion  of  saliva 
and  of  sweat.  To  these  centres  come  efferent  impulses 
from  all  parts  of  the  body,  impulses  which  may  never 
indeed  give  rise  to  conscious  sensation,  but  which,  acting 
on  the  nerve  centres  of  the  medulla,  so  stimulate  and  affect 
them  as  to  keep  them  constantly  ready  to  respond  to  the 
needs  of  the  organism.  Under  all  the  ordinary  circum- 
stances of  life,  whether  we  be  sleeping  or  waking,  these 
centres  pursue  the  even  tenor  of  their  way.  Influenced  by 
some  great  emotion,  at  some  great  crisis,  when  all  the 
energy  of  our  being  is  centred  upon  one  thought  or  one 
swift  effort,  these  centres  may  stand  in  abeyance  for  the 
moment ;  nay,  the  pang  may  be  so  great  that  the  vital  chain 
is  for  ever  broken,  but  as  a  rule  we  are  unconscious  even 
of  the  results  of  their  activity.  All  the  great  vital  functions 
go  on  unheeded,  unless  when  some  cause  arises  to  interfere 
with  their  free  and  unimpeded  action.  But  their  influence 
over  conscious  life  is  none  the  less  potent ;  without  their 


General  Introduction  19 

action  the  great  receptive  centres  of  the  brain  would  be 
powerless.  The  freedom  we  have  from  the  necessity  of 
consciously  watching  over  these  things  alone  renders  a 
higher  life  possible. 

2.  The  Medulla. — The  difficulties  experienced  in  ascer- 
taining the  paths  of  sensory  influences  in  the  cord  are  great, 
but  they  are  vastly  increased  when  we  come  to  examine 
the  medulla.  We  have,  in  fact,  to  depend  mainly  upon 
anatomical  and  pathological  research  for  what  little  we 
know,  and  it  is  only  possible  to  separate  certain  fibres 
which  we  can  positively  affirm  to  be  associated  with  motor 
functions. 

The  upward  bound  fibres  passing  through  the  medulla 
may  either  go  to  the  ganglia  at  the  base  of  the  brain,  to  the 
cerebellum  (Fig.  9,  B),  or  to  the  cerebral  hemispheres. 
A  complete  description  of  the  structure  and  functions  even  of 
the  parts  of  the  brain  devoted  in  the  main  to  the  sensory 
activities,  is  beyond  the  scope  of  the  present  work.  We 
can  only  attempt  to  give  a  mere  outline  of  the  cerebral 
mechanism. 

3.  The  Cerebellum. — The  cerebellum,  or  little  brain,  is 
connected  by  strands  of  nerve  fibres  both  with  the  cord  and 
with  the  brain  proper  ;  and  though  in  all  likelihood  it  acts  as  a 
co-ordinating  or  arranging  centre  for  the  nerve  currents  that 
induce  complicated  movements,  we  have  no  evidence  that 
it  contains  any  sensory  centres.  No  pain  is  felt  when  its  sub- 
stance is  injured,  nor  can  we  detect  any  alteration  in  general 
or  special  sensitivity.  Some  physiologists  have  advanced  the 
view  that  it  may  be  connected  with  the  muscular  sense. 
The  staggering  gait  and  irregular  movements  characteristic 
of  an  animal  whose  cerebellum  has  been  destroyed,  indicate 
a  loss  of  a  regulating  centre  which  normally  is  at  work. 
We  may  understand  this  if  we  reflect  for  a  moment  upon 
the    complicated   nature   of  the   movements    we  habitually 


20  Physiology  of  the  Senses 

perform.  Walking,  for  example,  involves  the  consensual 
action  of  many  groups  of  muscles,  each  of  which  must  act 
exactly  at  the  proper  time  and  with  most  delicately  adjusted 
force.  The  acquirement  of  the  power  is  only  gained  after 
many  attempts,  and  the  mere  preservation  of  the  upright 
attitude  of  the  body  is  only  possible  when  the  sensory 
impressions  from  the  feet  and  limbs  are  duly  transmitted 
and  take  their  place  in  the  complex  sum  of  afferent  impulses. 
Of  the  means  or  methods  by  which  the  multifarious  peri- 
pheral impressions  are  correlated,  and  after  the  nerve 
centres  are  excited,  the  adjustment  is  carried  out  and  the 
different  muscles  set  in  regulated  motion,  we  know  nothing. 
We  do  not  even  think  how  a  movement  is  to  be  made. 
We  simply  will  something  to  be  done,  and  it  is  done  ;  but  of 
the  intervening  causal  chain  we  are  quite  unconscious.  We 
think  of  the  end  and  not  of  the  means.  In  that  sense  our 
movements  are  automatic  ;  and  it  is  interesting  to  note  that 
the  more  any  given  movements  are  practised,  the  more  auto- 
matic they  become  ;  and  the  more  purely  automatic  they  are, 
the  more  accurately  are  they  adapted  to  their  aim.  Illustra- 
tions of  this  are  afforded  us  in  all  employments  where  a 
certain  small  piece  of  work  is  done  to  the  exclusion  of  all 
else.  The  hands  will  work  busily  while  the  thoughts  are 
far  away.  In  such  a  case  we  have  the  same  sensory  im- 
pression travelling  to  the  same  centre,  giving  rise  to  the 
same  outflow  of  energy,  and  along  the  same  efferent  channels, 
and  an  unconscious  memory  of  what  has  been  required  in 
the  past  enables  us  to  determine  without  effort  the  neces- 
sities of  the  present.  But  vary  the  surroundings  a  little,  and 
new  conscious  efforts  must  again  be  made,  and  the  work 
requires  longer  time  and  conscious  effort  and  attention.  It 
is  possible  that  the  necessary  fusion  of  impressions  takes 
place  in  the  ganglia  at  the  base  of  the  brain,  and  messages 
to  the  cerebellum  act  through  its  cells  and  fibres  as  through 


General  Introduction 


21 


distributing  centres  to  the  muscles  ;  but  of  this  we  cannot  at 
present  speak  with  certainty. 

4-   The  Pons. — The  medulla,  as  we  have  seen,  is  con- 


Fig.  8. — Base  of  the  brain,  i,  i,  The  longitudinal  fissure  dividing  the  hemi- 
spheres ;  2,  2',  2",  the  anterior  lobe  of  the  brain  ;  3,  fissure  of  Sylvius  ;  4,  4',  4", 
the  middle  lobe  of  the  brain  ;  5,  5',  posterior  lobe  ;  6,  bulb  or  medulla  oblon- 
gata ;  7,  8,  9,  10,  the  inferior  surface  of  the  cerebellum.  The  figures  I  to  IX 
indicate  cerebral  nerves  :  thus  I  is  the  olfactory  bulb  removed  on  the  right 
side ;  II  is  the  optic  nerve  with  decussation ;  V,  the  sensory  nerve  of  the 
face  and  part  of  the  scalp;  VII,  the  auditory  nerve;  VIII,  the  glosso- 
pharyngeal with  sensory  fibres  from  mouth  and  throat ;  III  is  on  a  crus 
cerebri ;  VI  and  VII  are  placed  on  the  Pons  Varolii ;  X,  the  first  nerve 
emergent  on  the  neck. 

nected  with  the  cerebellum  ;  the  rest  of  the  fibres  passing 
upwards  from  it  enter  a  structure  known  as  the  pons  Varolii^ 


22  Physiology  of  the  Senses 

or  bridge  of  Varolius  (Fig.  8,  VI,  VII ;  Fig.  9,  C),  so  called 
because  numerous  fibres  pass  through  it  from  one  side  of 
the  cerebellum  to  the  other,  and  these  form  a  transverse 
prominence  like  a  bridge  across  the  main  course  of  the 
nerve  fibres  which  pass  up  and  down.  In  the  pons,  as  in 
the  medulla,  we  find  many  nerve  centres  mixed  with  the 
fibres.  Here,  for  example,  among  others  are  situated  the 
centres  of  origin  of  the  great  nerve — the  fifth  cranial  (Fig. 
8,  V),  or  main  path  for  general  sensory  impressions  from 
the  face  and  scalp,  of  the  auditory  nerve  (Fig.  8,  VII) 
coming  from  the  ear,  and  of  the  nerves  which  control  the 
movements  of  the  muscles  of  the  face.  Fibres  carrying 
painful,  thermal,  and  tactile  impressions  probably  pass  up 
through  the  centre  of  the  pons,  where  also  some  of  them 
decussate.  The  motor  fibres  are  mainly  in  front  of,  and 
the  nerve  centres  behind,  these  thermal  and  tactile  paths. 

5.  The  Cerebrum. — Fibres  from  the  pons  and  cerebellum 
pass  to  the  cerebrum,  or  brain  proper,  by  the  connecting 
strands  known  as  the  cerebral  pedwtcles.  These  slope 
upwards  and  forwards,  and  the  anterior  and  lower  fibres 
branching  outward  as  they  enter  each  side  of  the  brain 
are  known  as  the  legs  of '  the  brain,  or  crura  cerebri. 
The  upper  and  back  part  of  the  peduncles  is  composed 
mainly  of  gray  matter,  and  when  seen  from  above  shows 
four  slight  elevations  known  as  the  corpora  quadrige- 
mina.  It  is  of  interest  to  note  that  the  corpora 
quadrigemina  receive  nerve  fibres  from  the  eyes  through 
the  optic  tracts,  and  are  concerned  in  the  mechanism 
of  vision.  Destruction  of  one  side  causes  blindness  in  the 
eye  of  the  opposite  side,  with  loss  of  power  of  accommoda- 
tion of  the  pupil  of  the  eye.  Whether  they  are  the 
seat  of  conscious  sensation  is,  however,  very  dubious. 
They  are  small,  and  hidden  away  under  the  superposed 
cerebral   mass   in   man,  but   the   corresponding   structures 


General  Introduction 


23 


known  as  the  optic  tobes  in  birds,  reptiles,  and  fishes  are 
large  and  important  relatively  to  the  rest  of  the  brain.  The 
most  attractive  hypothesis  is  that  they  act  in  man  as  centres 
for  the  fusion  of  impressions  coming  from  the  eyes  by  the 
separate  nerve  fibres,  and  for  the  regulation  of  bodily  or 
ocular  movements  dependent  upon  visual  impressions,  but 


Fig.  9. — Plan  in  outline  of  the  encephalon,  or  central  nerve  system  within  the 
skull,  as  seen  from  the  right  side.  A,  Cerebrum ;  B,  Cerebellum ;  C,  Pons 
Varolii ;  D,  Medulla  oblongata ;  a,  crus  cerebri  or  cerebral  peduncle  ;  b 
superior,  c  middle,  d  inferior  cerebellar  peduncles ;  b  is  placed  just  in 
front  of  the  corpora  quadrigemina  ;  e,  fissure  of  Sylvius  ;  f  anterior,  g 
middle,  h  posterior  lobes  of  cerebrum. 

that  for  conscious  vision  the  gray  matter  of  the  cerebrum 
must  be  likewise  affected. 

In  front  of  the  corpora  quadrigemijia,  and  lying  at  the 
base  of  the  brain,  lie  two  large  ganglionic  masses  -on  each 
side  of  the  middle  line — the  thalami  optici  and  the  corpora 
striata — between  which  passes  an  important  set  of  fibres 
from   the    crura,   known   as    the    internal   capsule.      Many 


24  Physiology  of  the  Senses 

sensory  fibres  are  believed  to  enter  the  optic  thalami, 
coming  either  by  way  of  the  corpora  quadrigemina,  the 
crura,  or  the  internal  capsule,  while  other  fibres  join  the 
thalami  with  the  cerebral  hemispheres.  From  their  con- 
nection with  the  corpora  quadrigemina  we  find,  as  might 
have  been  expected,  that  injury  to  the  optic  thalami,  more 
especially  in  their  hinder  parts,  causes  visual  disturbance, 
but  the  thalami  are  probably  connected  with  many  other 
sensory  fibres  besides  those  of  vision. 

The  human  brain,  when  stripped  of  its  investing  mem- 
branes and  viewed  from  above,  is  seen  to  consist  of  two 
masses  or  hemispheres  of  a  grayish  colour  externally,  a 
deep  furrow  running  between  the  hemispheres  from  before 
backward,  at  the  bottom  of  which  is  a  broad  band  of  white 
nerve  fibres,  the  corpus  callosum,  joining  the  two  masses. 
The  surface  is  not  smooth,  but  thrown  into  numerous  folds, 
convolutions,  or  gyri,  between  which  lie  depressions  of  vary- 
ing depth  called  sulci,  or  fissures.  Such  convolutions  are 
absent  from  the  brains  of  many  of  the  lower  forms  of 
animals,  and  even  in  man,  in  the  earliest  periods  of  life,  and 
they  are  present  in  the  adult  brain  in  order  to  allow  for 
increased  area  of  the  cerebral  surface  or  cortex.  At  a  first 
glance  these  ridges  and  furrows  seem  to  be  quite  irregular 
and  devoid  of  arrangement,  but  a  study  of  the  comparative 
appearances  of  many  human  brains  leads  us  to  see  that 
though  there  may  be  slight  divergencies  in  the  number, 
depth,  and  regularity  of  the  convolutions,  these  are  largely 
formed  on  the  same  plan.  We  see  that  the  brain  may  be 
regarded  as  made  up  of  several  lobes  (Fig.  10),  which 
are  named  according  to  the  part  of  the  cranium  in  which 
they  lie,  and  that  each  lobe  has  a  definite  number  of  ridges 
and  furrows,  the  names  of  which  are  given  in  the  explana- 
tion of  Fig.  i  o.  So  long  as  it  was  supposed  that  the  brain 
acted  as  a  whole,  and  that  no  special  function  was  associated 


General  Introduction 


25 


with  any  particular  area,  the  relationship  of  the  convolutions 
was   deemed   of  comparatively    little    importance.       Now, 


Fig.  10. — Semi-diagrammatic  view  of  the  left  side  of  the  brain.  F,  Frontal  lobe  ; 
P,  Parietal  lobe  ;  O,  Occipital  lobe  ;  T,  Temporo-sphenoidal  lobe  ;  S,  fissure 
of  Sylvius  ;  S'  horizontal,  S"  ascending  branch  of  the  same  ;  c,  central  sulcus 
or  fissure  of  Rolando  ;  A,  ascending  frontal ;  B,  ascending  parietal  convolu- 
tion ;  Fi,  Fo,  F3,  superior,  middle,  and  inferior  frontal  convolutions  \f\,f%, 
superior  and  inferior  frontal  sulcus ;  fz,  praecentral  sulcus ;  Pj,  superior 
parietal  lobule  ;  P2  supra-marginal  gyrus,  and  P2'  angular  gyrus,  parts  of 
inferior  parietal  lobule  ;  ip,  intra-parietal  sulcus  ;  an,  end  of  calloso-marginal 
fissure  (see  Fig.  n) ;  Oi,  0-2,  O3,  first,  second,  and  third  occipital  convolu- 
tions ;  po,  parieto-occipital  fissure  ;  0,  transverse  occipital  fissure  ;  o»,  inferior 
occipital  fissure  ;  Ti,  T2,  T3,  first,  second,  and  third  occipital  convolutions  ; 
t\,  t-2,  first  and  second  temporo-sphenoidal  fissures.     (Ecker.) 

however,  it  is  well  to  know  the  names  and  positions  tff  the 
various  lobes,  convolutions,  and  furrows,  so  as  to  be  able  to 


26  Physiology  of  the  Senses 

understand  descriptions  of  special  areas  of  the  surface. 
The  lobes  are  named  from  the  special  bones  of  the  skull 
with  which  they  come  into  contact,  and  are  known  respec- 
tively as  the  Frontal,  F,  the  Parietal,  P,  the  Occipital,  O, 
and  the  Temporo-sphenoidal,  T,  lobes.  It  will  be  seen 
by  reference  to  Fig.  10  that  there  are  two  specially  deep 
and  well-marked  fissures,  those  of  Rolando  (<f,  Fig.  i  o)  and 
of  Sylvius  (S,  S',  Fig.  10),  the  latter  of  which  is  branched,  S". 
To  the  front  are  three  well-marked  and  constant  ridges,  the 
frontal  gyri  (Fv  F2,  F3),  separated  by  two  furrows,  fv  f. 
In  front  of  the  fissure  of  Rolando  we  have  the  ascending 
frontal  convolution,  and  behind  it  the  ascending  parietal, 
behind  which  again,  and  separated  by  the  intra-parietal 
furrow,  lie  two  other  parietal  convolutions,  Pj  and  P2.  The 
second  parietal  convolution  becomes  continuous  with  the 
superior  of  three  temporal  convolutions,  Tv  T2,  and  T3,  by  a 
bend  round  the  end  of  the  Sylvian  fissure  immediately 
below  P2,  known  as  the  supra-marginal  convolution,  and  the 
superior  and  middle  temporal  convolutions  are  connected 
posteriorly  by  a  small  angular  convolution  at  P2',  commonly 
known  as  the  angular  gyrus.  Parts  of  three  occipital  con- 
volutions, Op  02,  03,  are  seen. 

Of  the  various  fissures  that  of  Sylvius  is  much  the  most 
marked,  the  others  being  merely  furrows.  The  Sylvian 
fissure  really  indicates  that  the  posterior  part  of  the  hemi- 
sphere has  in  the  process  of  development  been  bent  round 
and  packed  away  under  the  frontal  and  parietal  regions. 
When  the  Sylvian  fissure  is  opened  up  there  is  seen  a  small 
pyramidal  mass  of  gray  matter — the  island  of  Reil — the 
convolutions  of  whose  surface,  being  hidden  when  the  brain 
is  in  its  natural  state,  are  known  as  the  gyri  operti  The 
letter  S  lies  external  to  the  spot  in  which  these  convolu- 
tions are  to  be  found. 

When  the  two  hemispheres  are  separated  by  an  antcro- 


General  Introduction 


27 


posterior  section  in  the  median  plane  of  the  body,  each 
internal  surface  is  seen  to  present  certain  fissures  and  con- 
volutions, the  principal  of  which  are — (1)  the  marginal 
gyrus  F,3  which  is  really  the  internal  aspect  of  the  superior 
and  ascending  frontal  convolutions  and  ends  posteriorly 
at   the  fissure  of  Rolando  ;   (2)   the  gyrus  fomicatits,   Gf, 


Fig.  11. — Semi-diagrammatic  view  of  the  right  cerebral  hemisphere  in  its  median 
aspect.  CC,  corpus  callosum  divided  vertically  ;  G/J  gyrus  fornicatus  ;  H, 
gyrus  hippocampi  ;  h,  sulcus  hippocampi ;  U,  uncinate  gyrus ;  an,  calloso- 
marginal  fissure ;  F,  first  frontal  convolution ;  c,  end  of  central  sulcus 
(fissure  of  Rolando) ;  A,  ascending  frontal ;  B,  ascending  parietal  convolu- 
tions ;  Pi',  praecuneus  ;  po,  parieto-occipital  fissure  ;  o,  transverse  occipital 
sulcus  ;  Oz,  cuneus ;  oc,  calcarine  sulcus ;  oc',  oc",  its  superior  and  inferior 
branches ;  D,  descending  gyrus  ;  T4,  lateral  occipito-temporal  gyrus ;  T5, 
median  occipito-temporal  gyrus.     (Ecker.) 

separated  from  the  marginal  convolution  by  (3)  cm,  the 
callo  so  -marginal  fissure,  and  continuous  posteriorly  with 
(4)  H,  the  gyrus  hippocampus,  so  called  from  the  peculiar 
appearance  of  the  gray  matter  at  this  part.  It  is  con- 
tinued into  (5)  U,  the  uncinate  or  hooked-shaped  gyrus. 
P1  marks  the  internal  aspect  of  the  parietal  convolution,  or 


28  Physiology  of  the  Senses 

precuneus,  and  it  is  separated  by  po,  the  parieto-occipital 
fissure,  from  (6)  the  cunens  or  wedge-shaped  convolution, 
whose  lower  surface  is  separated  by  the  calcarine  fissure, 
oCj  from  (7)  the  temporo- occipital  convolutions •,  three  in 
number,  which  lie  at  the  base. 

Each  cerebral  hemisphere  has  a  central  cavity  or 
ventricle,  which  is  continuous  with  a  small  canal  that  passes 
through  nearly  the  entire  length  of  the  spinal  cord.  The 
internal  substance  of  each  side  of  the  brain  consists  of 
nerve  fibres  joining  the  surface  of  the  brain  with  the  lower 
nerve  centres,  or  one  part  of  the  brain  with  another.  The 
nerve  fibres  in  the  brain  have  only  the  axis -cylinder  and 
surrounding  white  substance  of  Schwann,  but  no  neuri- 
lemma, and  hence  the  difficulty  of  dissecting  out  special 
strands  of  nerve  fibres  and  of  tracing  the  course  they  run, 
is  considerably  increased.  The  fibres  which  have  been 
most  definitely  traced  are  (1)  those  from  the  internal 
capsule  (p.  23)  as  they  pass  outwards  to  the  cortex,  the 
group  called  the  corona  radiata ;  (2)  the  decussating  fibres 
of  the  corpus  callosum  ;  and  (3)  the  longitudinal  or  col- 
lateral fibres  connecting  different  parts  of  the  same  side  of 
the  brain. 

The  gray  matter  of  the  cortex  consists  of  nerve  cells 
and  fibres  embedded  in  a  connective  tissue  material,  the 
neuroglia,  and  well  supplied  with  blood  -  vessels  and 
lymphatic  channels.  The  cells  differ  slightly  from  each 
other  in  appearance  at  different  depths  from  the  surface 
and  in  different  areas,  and  may  be  of  a  pyriform,  conical, 
spindle-shaped,  or  quite  irregular  shape,  but  on  the  whole 
they  present  the  form  of  a  pyramid  whose  apex  points 
towards  the  surface  and  from  which  a  long  thin  pole  or 
fibre,  the  apex  process,  can  be  traced  outwards  for  some 
distance,  but  whose  ultimate  destination  is  unknown.  The 
base  of  the  pyramid  is  connected  with  a  nerve  fibre  coming 


General  Introduction  29 

from  the  subjacent  white  matter,  and  from  the  angles  at 
the  base  of  the  pyramid,  or  even  from  the  sides,  we  find 
numerous  branching  processes  —  in  some  cases  as  many 
as  fifteen  to  eighteen  —  the  number  of  which  seems  to 
depend  upon  the  size  and  age  of  the  cell.  These  processes 
are  short  and  quickly  break  up  into  a  fine  plexus  of  fibres, 
and  it  is  probable  that  these  act  as  internuncial  fibres 
bringing  the  different  cells  into  relationship  with  each 
other. 

The  general  arrangement  of  the  structures  in  the  cortex 
is  as  follows  : — On  the  surface  we  find  a  layer  of  nerve 
fibres  supported  by  fine  connective  tissue  and  vessels  pass- 
ing straight  inwards  to  reach  the  deeper  layers.  Next  to 
this  comes  a  layer  of  small  oval  or  angular  cells  with  large 
nuclei  and  giving  off  numerous  fine  processes.  Deeper 
down  comes  a  layer  containing  more  distinctly  pyramidal 
cells,  and  in  the  posterior  or  sensory  regions  we  find  many 
small  conical  cells  packed  together.  Below  these  again, 
we  find,  more  especially  in  the  motor  areas  (p.  30),  very 
large  pyramidal  cells  of  the  form  described  above.  Below 
these  again  come  a  set  of  spindle  cells  with  numerous 
nerve  fibres  passing  between  them  to  the  outer  cells. 

The  more  carefully  the  gray  matter  is  examined  the 
more  clearly  do  we  find  that  each  area  has  its  own  special 
groups  of  cells — a  rule  that  we  would  expect  to  hold  con- 
sidering the  varying  functions  of  the  different  areas  ;  never- 
theless the  transition  from  one  set  of  forms  to  another  is 
never  very  abrupt.  That  different  areas  of  the  brain  have 
different  functions,  though  often  conjectured,  was  not 
experimentally  proved  till  1870,  when  Fritsch  and  Hitzig 
performed  their  celebrated  experiments  ;  and  this  subject 
has  since  been  carefully  studied  by  many  observers,  among 
whom  we  may  mention  Ferrier,  Horsley,  and  Hitzig. 
Thus  it  has  been  established  that  the  convolutions  adjoin- 


30  Physiology  of  the  Senses 

ing  the  fissure  of  Rolando  have  to  do  with  the  initiation, 
under  due  stimulation,  of  movements  throughout  the  body, 
and,  generally  speaking,  the  broad  distinction  may  be  drawn 
that  the  frontal  and  front  part  of  the  parietal  lobes  are 
associated  either  with  the  exercise  of  the  more  purely  mental 
powers,  or  with  movement,  while  the  posterior  parietal  con- 
volutions and  the  occipital  and  temporo-sphenoidal  lobes 
have  to  do  with  sensation. 

The  sensory  fibres  to  the  occipital  and  temporo- 
sphenoidal  lobes  come  in  the  main  from  the  posterior  third 
of  the  internal  capsule,  spreading  outward  thence  in  a  fan- 
shaped  manner  as  the  radiation  of  Gratiolet. 

The  precise  position  of  the  different  centres  cannot  be 
precisely  stated,  but  by  localised  electrical  excitation,  or 
by  the  destruction  of  certain  areas  accidentally,  experi- 
mentally, or  by  disease,  "and  by  careful  observation  of  the 
variation  in  the  normal  phenomena  thus  caused,  the  fol- 
lowing tentative  conclusions  have  been  arrived  at  as  to  the 
sensory  centres  of  the  cortex.  Our  information  as  to  the 
centre  of  vision  is  more  definite  than  with  regard  to  the 
other  sensory  centres,  for  it  will  readily  be  understood 
that  blindness  is  much  more  easily  detected  in  an  animal 
than  the  loss  of  any  other  of  the  senses. 

Sensory  Centres  in  the  Cortex 

i.  The  Centre  for  Vision. — This  is  believed  to  lie  in  the 
convex  outer  surface  of  the  occipital  lobe  and  the  angular 
convolutions  (Fig.  10,  p.  25,  P2,  P2').  It  has  been  found  that 
electrical  stimulation  of  the  angular  gyrus  causes  the 
animal  to  turn  its  eyes  to  the  side  opposite  to  that  stimu- 
lated, and  upwards  or  downwards  according  as  the  front 
or  back  part  of  the  gyrus  is  excited.  Further,  the  eyelids 
are  closed  and  the  pupil  contracts.      What  is  the  meaning 


General  Introduction  31 

of  these  movements  ?  As  we  shall  see  in  dealing  with 
vision,  the  distribution  of  the  fibres  of  the  optic  nerve 
is  such  that  we  would  expect  that  the  occipital  lobes  of 
say  the  left  side  of  the  brain  would  take  cognisance  of 
everything"  visible  to  the  right  side  of  a  plane  passing  fore 
and  aft  through  the  body  when  the  eyes  are  looking 
straight  forward.  The  left  brain  has  to  do  with  fibres  from 
the  left  side  of  each  eye,  viz.  the  part  that  sees  objects 
to  the  right.  If,  then,  on  stimulating  the  left  occipital  or 
"  occipito-angular  "  area  the  eyes  turn  to  the  right,  we  may 
with  reason  interpret  the  movement  as  meaning  that  the 
stimulus  has  given  rise  to  the  sensation  of  something 
being  visible  in  the  right  half  of  the  field  of  vision  for  the 
better  view  of  which  the  head  is  turned  to  the  right,  while 
the  contraction  of  the  pupil  may  indicate  that  the  sensation 
is  of  something  near  at  hand,  and  the  closure  of  the  eyelid 
that  the  eye  is  shut  for  protection  from  contact  with  a 
near  object  or  the  shutting  out  of  a  too  brilliant  flash  of 
light. 

Destruction  of  the  central  part  of  the  occipito-angular  area 
causes  disturbance  of  vision  or  blindness  of  the  same  side 
of  each  retina,  or,  in  other  words,  for  the  opposite  side 
of  each  visual  field.  But  an  important  and  delicate  dis- 
tinction must  be  drawn.  The  blindness  is,  according  to 
Munk,  one  of  mind — "  a  psychical  blindness  or  inability  to 
form  an  intelligent  comprehension  of  the  visual  impressions 
received."  The  eye  performs  its  function  correctly ;  the 
basal  ganglia  may  fuse  the  sensations  into  a  coherent 
whole,  the  animal  may  act  in  a  reflex  way  avoiding 
obstacles  in  its  path,  but  the  object  thus  seen  awakes  no 
mental  activity.  An  example  will  illustrate  our  meaning. 
It  can  see  and  avoid  as  it  walks  a  plate  containing  food, 
but  it  does  not  recognise  food  as  such,  as  something  to  eat, 
nor  does  it  show  signs  of  fear  when  threatened  with  a  whip. 


32  Physiology  of  the  Senses 

It  has  been  suggested  that  the  removal  of  the  central  part 
of  the  occipital  lobe  merely  removes  that  part  of  the  cortex 
which  is  associated  with  the  area  of  distinct  vision  of 
the  retina,  that  the  animal  has  conscious  but  not  distinct 
vision.  This  would  be  in  agreement  with  the  fact  that 
when  only  injured  upon  one  side  the  animal  within  a  few 
days  recovers  to  some  extent  the  sense  of  sight  on  the 
side  affected.  The  improvement  might  be  due  to  acquisi- 
tion by  practice  of  powers  of  vision  not  usually  possessed 
by  the  peripheral  parts  of  the  retina,  but  much  has  still  to 
be  learned  on  this  difficult  subject.  Complete  destruction 
of  the  occipito-angular  areas  of  both  sides,  the  cuneus  (Fig. 
ii,  Oz)  being  included,  causes  total  and  permanent  blind- 
ness without  any  other  perceptible  loss  of  sensory  or  motor 
power. 

The  power  of  distinct  vision,  then,  depends  in  man 
upon  the  normal  working  of  a  terminal  organ,  the  eye,  of 
the  optic  nerve  partially  decussating  at  the  optic  com- 
missure, the  nerve  strands  passing  thence  backwards  by 
the  optic  tract  to  the  corpora  quadrigemina  and  optic 
thalami,  and  thence,  by  the  radiation  of  Gratiolet,  to  the 
cortex  of  the  posterior  part  of  brain.  We  have  seen  it  to 
be  the  law  that  when  a  nerve  fibre  is  cut  off  from  its  gan- 
glionic nerve  cell  the  fibre  degenerates.  In  the  case  of 
the  optic  mechanism,  these  ganglionic  cells  are  situated  in 
the  retina,  which  the  study  of  development  has  shown  to 
be  really  a  part  of  the  brain,  and  when  the  retina  is 
destroyed  the  optic  fibres  passing  from  it  undergo  de- 
generation. 

2.  The  centre  of  hearing  for  each  ear  seems  to  be 
situated  in  the  superior  temporal  convolution  (Fig.  10,  Tj) 
of  the  opposite  side.  The  fibres  of  the  auditory  nerve 
after  entering  the  medulla  pass  upwards  through  the  pons, 
decussate  there,  and  thence  go  through  the  posterior  part 


General  Introduction  33 

of  the  internal  capsule  to  the  temporal  region.  Our  most 
valuable  evidence  as  to  the  auditory  centre  comes  from 
cases  where  the  brains  of  deaf  or  epileptic  patients  have 
been  examined  post-?nortem.  Thus  in  certain  instances 
the  cause  of  deafness  has  been  found  to  be  disease  of  the 
above  -  mentioned  area ;  and  in  cases  of  epilepsy  where 
the  fit  has  been  preceded  by  the  sensation  of  a  noise  the 
seat  of  disease  has  been  in  the  neighbourhood  of  this  part, 
and  the  irritation  thus  arising  has  determined  the  onset 
first  of  auditory,  and  then  of  motor,  disturbance  in  the 
adjoining  motor  areas.  The  study  of  peculiar  sensory 
disturbances  which  often  precede  a  convulsive  attack,  the 
mirci)  as  it  is  called,  is  of  great  interest  in  this  connection 
as  showing  the  part  of  the  brain  first  affected  by  the  dis- 
turbing force.  Most  commonly  it  is  an  indescribable 
sensation  seeming  to  originate  in  the  limbs  or  body  and 
passing  upwards  to  the  head,  and  that  in  many  cases  so 
slowly  as  to  be  capable  of  being  arrested  by  pressure.  In 
such  cases  it  is  most  probably  due  to  the  disturbance  of 
the  muscular  sense,  but  sometimes  the  aura  takes  the  form 
of  a  flash  of  light,  a  noise,  a  disagreeable  odour  or  peculiar 
taste,  in  which  case  the  centres  of  special  sense  are  the  parts 
more  directly  affected.  Fortunately  for  man,  epileptic 
attacks  are  seldom  directly,  and  in  the  earliest  stages  of 
the  disease,  fatal,  and  our  knowledge  of  the  intimate 
structure  of  the  brain  has  been  so  recently  acquired,  that 
pathological  investigation  has  not  been  of  so  much  service 
as  might  be  supposed.  There  is  undoubtedly  reason  to 
believe  that  this  branch  of  study  will  yield  fruitful  results 
in  future. 

Electrical  stimulation  of  the  corresponding  area  in  the 
dog  causes  pricking  up  of  the  opposite  ear,  turning  of  the 
eyes  and  head  to  the  opposite  side,  with  the  pupils  of  the 
eyes  dilated,  movements  such  as  the  dog  would  make  were 

D 


34  Physiology  of  the  Senses 

it  to  hear  a  sudden  sound  from  the  side  opposite  to  that 
stimulated. 

Destruction  of  the  superior  temporal  convolution  causes, 
according  to  Ferrier  and  others,  deafness  in  the  opposite 
ear,  but  this  is  denied  by  Schafer  and  Horsley,  who  urge 
the  difficulty  of  determining  the  presence  of  deafness,  and 
maintain  that  in  one  case  where  both  temporal  lobes  were 
completely  destroyed  there  was  no  perceptible  loss  of  the 
power  of  hearing.  In  the  case  of  human  beings  it  is 
believed  that  there  may  be  only  a  partial  decussation  of  the 
nerves  of  hearing,  just  as  in  the  case  of  sight,  so  that  injury 
to  one  side  of  the  brain  may  not  cause  complete  loss  of 
hearing  on  either  side,  but  where  both  sides  have  been 
affected  the  loss  of  hearing  is  complete.  With  hearing  as 
with  sight  Munk  believes  there  may  be  a  psychical  as 
opposed  to  a  complete  loss  of  sensation,  and  he  affirms  that 
destruction  of  the  middle  part  of  the  convolution  causes 
psychical  deafness. 

3  and  4.  The  centres  for  taste  and  smell  are  supposed 
by  Ferrier  to  be  situated  in  the  anterior  part  of  the  hippo- 
campal  or  uncinate  gyri  (Fig.  11,  H,  U),  as  indicated  by 
movements  of  the  nose  and  lips  on  stimulation  of  these 
areas.  The  nerves  of  smell  pass  upwards  from  the  nose  to 
the  olfactory  lobes,  which  lie  in  man  below,  and  covered 
completely  by,  the  frontal  lobes,  though  in  many  of  the  lower 
animals  they  are  prominent  bodies  projecting  forward 
beyond  the  rest  of  the  brain.  Fibres  from  the  olfactory 
lobes  have  been  traced  to  the  region  above  indicated,  but 
with  regard  to  their  ultimate  distribution,  and  still  more  to 
that  of  the  nerves  of  taste,  there  is  much  to  be  yet  learned. 
A  case  is  recorded  of  an  epileptic  patient  whose  aura  was 
of  an  olfactory  kind,  and  the  seat  of  disease  was  found  to  be 
in  the  right  uncinate  gyrus. 

5.   The  centre  for  touch  is  believed  by  Ferrier  to  be 


General  Introduction  35 

situated  in  part  at  least  in  the  gyrus  hippocampus,  as  shown 
by  loss  of  tactile  sensibility  when  this  area  is  destroyed. 

Schafer  and  Horsley  found  temporary  loss  of  sensation 
on  the  opposite  side  of  the  body — hemi-anaesthesia — when 
this  part  was  destroyed,  but  the  loss  was  more  marked  and 
persistent  when  the  greater  part  of  the  gyrus  fornicatus  (Fig. 
1 1,  Gf,  p.  27)  was  destroyed.  It  has  not  been  possible  to  find 
separate  centres  for  painful  and  tactile  impressions,  although 
from  considerations  advanced  when  speaking  of  the  sensory 
tracts  in  the  cord  it  is  quite  probable  that  such  do  really 
exist. 

It  is  only  of  late  years  that  an  attempt  has  been  made 
to  distinguish  between  sensory  terminations  for  the  per- 
ception of  heat  and  cotd,  and  no  observation  has  yet  been 
made  as  to  the  localisation  of  corresponding  sensory  areas 
in  the  brain.  It  has  been  found  that  injury  to  the  basal 
ganglia,  and  more  especially  to  the  corpus  striatum,  is 
followed  by  a  prolonged  rise  in  temperature,  as  if  a  centre 
which  had  normally  to  do  with  the  regulation  of  temperature 
had  been  affected ;  but  this  is  not  known  to  be  associated 
with  any  sensory  effects,  and  indeed  it  would  be  hard  to 
distinguish  experimentally,  except  upon  one's  self,  between 
sensations  of  touch,  of  pain,  and  of  variation  in  temperature. 

In  addition  to  the  special  forms  of  sensation  we  have 
just  considered,  there  are  many  sensations  of  a  general  kind 
— common  sensations — arising  from  the  internal  conditions 
of  the  body,  such  as  hunger,  thirst,  lassitude,  the  feelings 
due  to  distension  of  the  viscera,  and  many  peculiar  sensa- 
tions due  to  disturbance  of  the  nervous  system,  such  as  those 
felt  when  a  limb  is  said  to  be  asleep,  or  formication,  the 
condition  in  which  it  seems  as  if  ants  were  creeping  about 
under  the  skin.  Again  these  and  even  vaguer  conditions 
arising  from  varying  general  nutrition,  such  as  the  feeling  of 


36  Physiology  of  the  Senses 

general  well-being,  and  its  opposite,  discomfort,  general 
depression,  or  melancholy,  or  the  restless  condition  caused 
in  many  by  the  disturbance  of  the  electrical  condition  of  the 
atmosphere  usually  preceding  a  thunderstorm.  For  all 
these  no  special  cerebral  centres  have  been  found. 

The  Muscular  Sense. — Some  at  least  of  these  may 
probably  be  regarded  as  special  forms  of  the  muscular  sense, 
that  is  to  say  of  that  sensation  by  which  we  are  aware  of 
the  position  and  state  of  relaxation  or  contraction  of  the 
muscular  system  of  the  body,  and  by  which  we  are  guided 
in  our  unconscious  estimate  as  to  the  amount  of  force 
necessary  for  movement.  Through  it,  too,  we  can  estimate 
the  relative  weights  of  bodies. 

Relation  of  Stimulus  and  Sensation 

We  have  now  to  consider  in  general  terms  the  effect  Upon 
the  sensorium  which  any  given  change  in  our  environment 
or  in  the  body  itself  will  bring  about.  This  may  be  viewed 
from  two  aspects,  the  qualitative  and  the  quantitative. 

Qualitatively,  the  effect  will  depend  upon  whether  a 
special  or  a  common  sensory  mechanism  is  affected.  If  the 
stimulus  be  one  fitted  to  excite  the  sense  of  taste,  the 
sensation  it  causes  is  in  no  way  comparable  qualitatively  to 
that  caused  by  excitation  of  the  sense  of  vision.  The 
variation  of  quality  within  the  limits  of  any  one  of  the 
senses  varies  with  the  peculiar  nature  of  the  excitant.  The 
quality  of  colour,  e.g.,  varies  with  the  wave-length  of  light, 
or,  in  other  words,  with  its  rapidity  ;  that  of  sound  with  the 
form  of  wave,  or  more  accurately  with  the  momentum  of 
impact  or  pressure  on  the  sensory  apparatus ;  that  of  taste 
and  smell  with  the  molecular  constitution  of  the  body,  but 
whether  through  the  rate  of  motion  of  the  molecule,  or  the 
form  of  the  path  in  which  it  moves,  cannot  be  said.      Special 


General  Introduction  37 

illustrations  of  this  will  be  found  in  the  chapters  upon  the 
special  senses.  Quantitatively,  the  character  of  the  sensa- 
tion depends  upon  the  receptivity  of  the  organism  and  the 
amount  or  strength  of  the  stimulus.  The  stimulus  may  be 
so  feeble  that  it  fails  to  arouse  any  sensation  whatever,  a 
light  may  be  so  small  or  so  far  removed  from  the  eye  as  to 
be  invisible,  a  sound  may  be  so  faint  as  to  be  inaudible. 
But  when  the  energy  of  the  physical  disturbance  reaches  a 
certain  degree,  supposing  that  the  receptivity  of  the  sensory 
organ  is  always  the  same,  a  sensation  is  felt.  Other  things 
being  equal,  the  amount  of  energy  required  for  the  stimu- 
lation of  any  given  sense  may  be  regarded  as  a  constant 
quantity,  and  the  smallest  perceptible  amount  is  known  as 
the  lower  limit  of  excitation.  This  excitant  acting  on  the 
sensory  organ  brings  us,  as  it  were,  to  the  threshold  of 
sensation.  In  estimating  the  comparative  intensities  or 
strengths  of  sensations  it  is  commonly  assumed  that  the 
difference  between  zero  or  absence  of  excitant  and  the  lower 
limit  of  excitation  may  be  regarded  as  the  unit  of  measure- 
ment. 

We  say,  for  example,  that  lights  from  various  sources,  as 
a  candle,  an  oil  lamp,  an  electric  light,  the  sun,  have 
different  degrees  of  brilliancy  or  intensity.  We  may 
diminish  the  brightness  till  we  reach  a  point  beyond  which 
the  light  is  no  longer  seen,  and  yet  there  is  a  certain 
amount  of  energy  being  exerted  of  which  our  senses  fail  to 
take  cognisance.  In  stating  the  relative  brilliancies  or 
intensities  of  the  light  we  would  use  as  a  unit  of  comparison 
the  amount  of  light  just  sufficient  to  give  a  sense  of  lumin- 
osity. Then  so  many  times  this  unit  would  give  the  measure 
of  the  luminosity  of  the  candle,  so  many  more  of  the  lamp- 
light, so  many  more  of  the  electric  light  or  of  sunlight. 
We  may  say  that  the  intensity  of  one  sensation  is  double, 
treble,  quadruple   that  of  another,   and   so  on  ;  or,  on  the 


38  Physiology  of  the  Senses 

other  hand,  we  may  say  that  a  given  amount  of  sensation 
always  bears  a  certain  ratio  to  the  least  perceptible  differ- 
ence from  it,  either  in  the  way  of  increase  or  diminution. 
This  ratio,  again,  corresponds  with  that  between  the  in- 
tensities of  the  excitant  and  the  sensation.  An  endeavour 
has  been  made  to  put  this  latter  ratio  upon  an  absolute 
basis  for  each  of  the  senses,  but  this  can  only  be  stated  as 
an  average  of  a  number  of  determinations  made  by  different 
individuals  or  by  the  same  individual  at  different  times. 
Thus,  for  example,  it  is  stated  that  the  least  possible  differ- 
ence in  the  intensity  of  light  which  will  allow  of  a  sense 
of  different  luminosity  is  -~j.  Given  I  oo  lights,  a  difference 
of  luminosity  would  be  noted  if  one  were  added  or  with- 
drawn ;  but,  given  a  thousand,  no  difference  would  be 
observed  unless  at  least  10  were  added  or  removed.  The 
least  perceptible  difference  of  pressure  is  caused  by  the 
increase  or  diminution  by  ^  of  the  original  amount.  If  a 
person  is  holding  three  pounds  in  his  hand  he  will  not  feel 
any  increase  or  diminution  of  their  original  weight  unless 
as  much  as  one  pound  is  added  or  subtracted.  For  the 
pressure  sense  the  ratio  i  :  3  is  a  constant,  whatever  be 
the  original  unit.  Similarly  for  the  other  senses,  the  ratio 
for  the  sensation  of  temperature  is  1  :  3,  for  auditory  sensa- 
tions 1  :  3,  for  muscular  sensation  6  :  100,  and  for  visual 
sensation  1  :  100. 

In  the  next  place,  we  must  note  that  with  variation  in 
the  amount  of  the  stimulus  there  is  variation  in  the  intensity 
of  the  stimulation,  but  these  do  not  vary  pari  passu 
in  the  same  numerical  ratio.  We  have  seen,  for  ex- 
ample, that  to  have  any  change  at  all  in  the  sense 
pressure  there  must  be  an  increase  or  diminution  by 
1  of  the  original  pressure,  but  we  do  not  necessarily 
recognise  directly  that  the  pressure  is  ^  more  or  less.  The 
law  only  holds  that  there  will  be  an  equal  perceptible  varia- 


General  Introduction  39 

tion  when  the  stimulus  varies  in  constant  proportion. 
There  is  the  same  perceptible  variation  when  3  lbs.  are 
increased  to  4  lbs.,  as  when  6  are  changed  to  8,  or  12  to 
16.  Fechner  points  out  that  the  strength  of  a  sensation 
does  not  increase  in  the  same  numerical  ratio  with  the 
strength  of  the  stimulus,  but  as  the  logarithm  of  the 
strength  of  the  stimulus,  for  logarithms  of  numbers  increase 
by  equal  increments  according  to  the  relative  increase  of 
the  numbers  themselves.  Thus  1,  2,  3,  etc.,  are  the 
logarithms  of  10,  100,  1000,  and  similarly,  the  increase  in 
sensation  when  the  excitant  is  increased  from  10  to  100 
will  be  the  same  as  when  the  100  are  increased  to  1000. 
Or,  putting  it  in  another  way,  the  strength  of  the  sensation 
increases  in  numerical  progression  as  the  strength  of  the 
stimulus  increases  in  geometrical  progression.  This  law, 
however,  only  holds  within  certain  limits — between  the 
threshold  of  sensation  on  the  one  hand,  and  an  upper  limit 
on  the  other.  With  all  sensations  there  comes  a  time  when 
an  increase  in  the  strength  of  the  stimulus  no  longer 
increases  the  intensity  of  the  sensation,  but  gives  rise  to  a 
change  in  quality.  Thus  beyond  a  certain  degree  of 
brilliancy  the  eye  will  be  blinded  or  rendered  insensitive  to 
light,  with  sounds  too  loud  the  ear  will  be  deafened,  with 
too  great  pressure  the  tissues  will  be  crushed,  and  with 
injury  to  the  sensory  organ  the  sense  of  pain  arises. 
Fechner's  law,  again,  fails  in  its  applicability  to  the  senses 
of  taste  and  smell,  and,  except  within  narrow  limits,  to  the 
sense  of  temperature,  while  it  holds  best  perhaps  in  regard 
to  the  sensation  of  light,  where,  owing  to  the  delicacy  of 
discrimination  of  the  sense  of  vision,  it  is  possible  to  judge 
of  differences  over  a  wide  range  of  sensibility. 

Sensations  and  Perceptions. — There  is  still  one  point 
in  which  we  may  note  a  difference  in  the  mental  effect  of 
the  action  of  the  different  senses,  viz.  the  extent  to  which 


40  Physiology  of  the  Senses 

they  are  attended  by  the  idea  of  externality.  With  both 
sight  and  hearing  we  very  early  acquire  the  power  of  pro- 
jecting our  sensations  outwards,  so  that  objects  seen  are 
referred  to  their  relative  positions  in  space,  while  by  the 
aid  of  other  senses  we  are  able  to  refer  the  sound  to  the 
sounding  body.  Similarly  we  refer  odours  to  the  body  from 
which  they  come,  and  the  senses  of  touch  and  taste  give 
us  information  which  we  interpret  as  due  to  objects  in  con- 
tact with  our  body,  but  external  to  it.  The  common  senses, 
such  as  fatigue,  pain,  etc.,  give  us  no  impression  of  an 
external  body  in  relation  to  ours,  they  are  purely  feelings 
devoid  of  a  sense  of  an  underlying  objective  reality.  This 
aspect  of  the  subject  will  be  better  understood,  however, 
when  we  consider  the  senses  in  detail. 


THE   SENSE   OF   TOUCH 


The  sense  of  touch  is  located  in  the  skin.  The  structure  of 
this  organ,  which  acts  as  a  protective  covering,  and  is  also 
concerned  in  the  excretion  of  sweat,  oily  or  sebaceous 
matter,  and  gases,  and  in  the  regulation  of  the  heat  of  the 
body,  will  be  readily  understood  by  studying  the  section 
seen  in  Fig.  12. 


(  Horny  layer  . 
Epi-     J 
dermis,    j  Clear  layer     . 
^Mucous  layer 


True 
Skin. 


Papillary  layer  .     . 
Duct  of  sweat  gland 

^  Reticulated  layer    . 
Sweat  gland  .     .     . 

Subcutaneous  tissue 


Fig.  12. — Perpendicular  section  of  the  skin  of  the  finger  of  an  adult  man. 
Magnified  15  diameters.     (Stohr.) 

Structure  of  the  Skin. — It  consists  of  two  layers,  a 
deeper,  formed  of  connective  tissue,  and  called  the  derma, 
cutis  vera  (true  skin),  or  corium,  and  a  superficial,  known 
as  the  epidermis,  which  is  composed  of  epithelium. 


42  Physiology  of  the  Senses 

(i)  The  true  skin. — If  we  look  at  the  surface  of  the 
skin  we  see  it  shows  delicate  furrows  or  grooves  crossing 
each  other,  so  as  to  form  small  lozenge-shaped  areas,  or 
the  grooves  may  run  parallel  for  a  considerable  distance. 
The  lozenge-shaped  arrangement  is  seen  on  the  surface  of 
the  skin  of  the  arm,  and  that  with  the  grooves  forms  a 
marked  feature  on  the  skin  of  the  palm  or  covering  the 
tips  of  the  fingers.  On  the  summits  of  the  ridges,  on  each 
side  of  a  groove,  or  enclosing  a  lozenge-shaped  area,  we 
find  small  prominences  termed  ftaftill<z,  the  number  and 
size  of  which  vary  much  in  different  parts  of  the  skin. 
They  are  most  numerous,  and  attain  the  greatest  size 
(about  the  T|-g  of  an  inch  in  length),  in  the  palm  of  the 
hand  and  sole  of  the  foot,  while  they  are  much  smaller  and 
fewer  in  number  on  the  skin  of  the  cheeks  or  forehead. 
The  true  skin  is  formed  of  a  felt-work  of  connective  tissue, . 
mixed  with  elastic  fibres,  and  having  also  a  considerable 
number  of  smooth  muscular  fibres  distributed  here  and 
there.  In  the  upper  layers  the  connective  tissue  is  con- 
densed so  as  to  form  a  firm  stratum,  but  in  the  deeper 
layers  the  bands  of  connective  tissue  run  in  all  directions 
so  as  to  form  an  irregular  mesh-work,  in  the  spaces  of 
which  we  find  numerous  fat-cells.  Thus  the  skin  is  toler- 
ably firm  in  its  upper  layers,  and  these  may  be  supposed  to 
rest  on  an  elastic  cushion,  a  condition  that  favours,  as  we 
will  find,  the  mechanism  of  touch. 

(2)  The  Epidermis. — This,  the  outermost  layer,  is  formed 
of  more  or  less  flattened  epithelial  cells,  arranged  in  layers 
or  strata.  Two  such  strata  are  readily  seen  when  we 
examine  a  perpendicular  section  (Fig.  12):  a  deeper 
stratum,  of  soft  consistence,  filling  up  the  spaces  between 
the  papillae,  and  termed  the  stratum  miicosiim,  or  stratum 
of  Malpighi  (after  the  Italian  anatomist  who  first  described 
it),  and   a   superficial   and  denser   stratum,  known  as  the 


The  Sense  of  Touch  43 

horny  lay^er  or  stratum  corneum.  Both  strata  are  built  up 
of  epithelial  cells,  which  change  in  appearance  as  we  pass 
from  below  upwards.  Those  in  the  mucous  stratum  are 
cylindrical,  and  have  a  long  nucleus ;  and  above  these 
we  find  rounded  cells,  having  little  spines  projecting  from 
their  borders,  and  hence  called  prickle  cells.  The  spines  of 
adjoining  cells  unite,  and  thus  there  is  a  reticulated  space 
round  each  cell.  Above  these  the  cells  become  more 
flattened,  and  contain  bright  refractive  granules.  The  cells  in 
the  mucous  layer  of  the  skin  rapidly  multiply,  the  youngest 
cells  being  next  the  papillae  of  the  true  skin,  and  each 
layer  is  gradually  pushed  towards  the  surface  by  a  layer  of 
younger  cells  below  it.  The  horny  layer  is  formed  of  flat 
polygonal  cells  that  have  lost  their  nucleus,  and  the  cells 
of  the  most  superficial  layer  are  gradually  being  shed  by 
abrasion  or  rubbing.  Thus  thousands  of  hard  dry  epithelial 
cells  are  being  rubbed  off  daily  from  the  surface  of  the 
epidermis.  In  some  parts  of  the  skin  where  the  epidermis 
is  very  thick,  as  on  the  sole  of  the  foot,  a  clear  stripe  is 
seen  between  the  mucous  and  horny  layers.  This,  called 
the  clear  stratum  {stratum  hicidum),  is  formed  of  cells  that 
refract  light  strongly,  and  hence  have  a  translucent  appear- 
ance. The  colour  of  the  skin  depends  partly  on  granules 
of  pigment  found  in  the  cells  of  the  mucous  layer,  and 
partly  on  the  blood  circulating  through  it,  and  the  thickness 
of  the  layer  of  tissue  between  the  vessels  and  the  surface. 
Thus  when  the  vessels  of  the  skin  are  moderately  dilated, 
and  when  the  vessels  lie  near  the  surface,  there  may  be  the 
delicate  rosy  hue  of  health,  while  the  reverse  conditions 
may  produce  a  pale  or  swarthy,  or  even  a  yellowish  tint  of 
skin. 

It  is  foreign  to  the  purpose  of  this  work  to  describe 
all  the  so-called  appendages  of  the  sh'n,  such  as  nails, 
hairs,  horn,  hoof,  quills,  feathers,  and  scales.     And  yet  all 


44  Physiology  of  the  Senses 

these  may  be,  to  some  extent,  concerned  in  the  sense  of 
touch.  They  are  all  modifications  of  epidermis,  and  they 
are  all  developed  or  moulded  upon  papillae  which  are 
similar  in  character  and  origin  (although  often  much 
greater  in  size)  to  the  papillae  on  the  surface  of  the  true 
skin  already  described.  The  following  general  statements 
regarding  these  appendages  are  of  physiological  interest : — 
(i)  Each  epidermic  structure  may  be  regarded  as  a 
permanent  excretion.  They  are  separated  from  the  blood, 
and  thus  modify  the  constitution  of  that  fluid.  Thus  the 
nutrition  of  other  organs  of  the  body  may  be  influenced, 
and  in  this  way  we  may  establish  a  physiological  connec- 
tion between  the  development  of  hairs,  horns,  wattles, 
combs,  brilliantly-coloured  feathers,  etc.,  and  the  changes 
at  'certain  periods  of  life  in  the  sexual  organs. 

(2)  Each  epidermic  structure  has  an  individual  exist- 
ence ;  it  is  developed,  grows,  reaches  maturity,  declines, 
dies,  and  is  removed  from  the  body,  to  be  replaced  by 
another  of  a  similar  kind.  Thus  hairs,  nails,  feathers,  etc., 
have  each  a  limited  duration  of  life. 

(3)  Epidermic  structures,  similar  in  origin,  but,  in  their 
mature  condition,  very  different  in  structure,  may  serve 
purposes  of  beauty,  as  the  hairs  of  the  seal  or  ermine,  the 
feathers  of  the  humming-bird  or  kingfisher,  or  the  scales  of 
the  gold-fish  or  mackerel ;  of  warmth,  as  the  hair  of  the 
polar  bear,  the  wool  of  sheep,  and  the  feathers  of  many 
birds  ;  of  defence,  as  the  horns  of  the  stag,  the  spines  of 
the  hedgehog,  or  the  quills  of  the  porcupine  ;  as  protect- 
ive and  resistant  structures,  covering  delicate  parts  of  the 
foot,  as  the  hoofs  of  the  horse,  etc. ;  and  as  aids  to  the  sense 
of  touch,  as  the  whiskers  of  the  cat,  or  the  hairs  on  the  ears 
of  many  nocturnal  mammals.  It  is  remarkable  that  when 
epidermis  is  modified  for  purposes  requiring  great  powers  of 
resistance,  it  assumes  in  structure  a  concentric  arrangement 


The  Sense  of  Touch  45 

of  epidermic  cells,  simulating  bone,  as  may  be  seen  by 
comparing  a  section  of  bone  with  that  of  hoof,  whalebone, 
or  of  rhinoceros  horn.  Lastly,  epidermic  structures,  by 
containing  pigment,  confer  brilliant  colours  on  many 
animals,  and  even  where  pigment  is  absent,  beautiful 
iridescent  tints  may  be  produced  by  fine  markings  on  the 
surfaces  of  epidermic  structures.  These  markings,  or 
grooves,  form  diffraction  spectra  when  the  light  falls  on 
them,  and  thus  we  have  many  humming  -  birds  flashing 
a  variety  of  tints  as  the  animals  flit  to  and  fro  in  the  sun- 
light. 

Structure  of  Tactile  Organs. — As  already  explained, 
sensory  nerves  are  those  that  convey  nervous  impulses  to 
the  brain,  and  there  give  rise  to  sensations.  Such  sensory 
nerves  abound  in  the  skin,  but  if  one  of  these  be  gently 
touched,  the  result  will  not  be  a  sensation  of  touch  in  the 
proper  sense  of  the  word,  but  a  more  or  less  painful  and 
disagreeable  impression.  The  direct  contact  of  any  foreign 
body  with  a  naked  sensory  nerve  is  too  rude  a  form  of 
stimulation,  and  hence  we  find,  as  a  rule,  that  the  fine  fila- 
ments at  the  origins  of  such  nerves  in  the  skin  are  brought 
into  relation  with  special  tactile  structures  or  terminal  organs 
of  touch,  of  which  there  are  several  varieties. 

(1)  Free  nerve -endings. — In  a  few  situations,  single 
nerve  fibres  pass  up  to  the  under  surface  of  the  epidermis, 
lose  their  medullary  sheaths,  and  then  the  axis -cylinder 
subdivides  into  fine  filaments,  which  either  lose  themselves 
among  the  cells  of  the  epidermis,  or  come  into  contact  with 
cells  having  branched  processes,  called  the  cells  of  Langer- 
hans.  This  is  the  simplest  form  of  nerve-ending,  and  the 
only  form  in  epidermis.  It  has  been  found  in  the  cornea 
of  the  eye,  the  nose  of  the  mole,  the  nose  of  the  pig,  and 
the  skin  of  the  frog  and  tadpole. 

(2)  Nerve-endings  in  corpuscles. — The  nerve  filaments 


46 


Physiology  of  the  Senses 


a- if 


may  terminate  in  various  forms  of  corpuscles,  which,  how- 
ever, are  (with  one  exception)  situated  in  the  true  skin,  or 
in  the  subcutaneous  tissue.  Thus  we  may  have  (a)  simple 
tactile  cells ;  (b)  groups  of  tactile  cells ;  (c)  touch  cor- 
puscles, (a)  simple,  and  (ft)  compound;  (d)  end- bulbs  ; 
and  (e)  a  more  complicated  structure  called  a  Pacinian 
corpuscle. 

(a  and  b)  Simple  tactile  cells. — These  are  oval  nucleated 
cells,  about  y-gVo  °f  an  mc^  *n  diameter,  found  in  the 
deeper  layers  of  the  epidermis,  or  in  the  true  skin  close 

to  the  epidermis.  Minute 
nerve  filaments  terminate 
in  these  by  apparently 
blending  with  their  sub- 
stance. Sometimes  these 
cells  may  form  a  group 
which  takes  the  form  of  a 
little  cup,  like  a  wine-glass 

Fig.  13.— Vertical  section  through  the  skin  with  the  bottom  broken  off, 

covering  the  attached  end  of  the  upper  ^      nerve    ending     in     the 

mandible  of  a  goose.       Magnified  240  '           ° 

diameters.     Shows  two  touch  corpuscles  Stem  of  the  glass, 

divided  transversely  to  the  plane  of  en-  (          *  $..;    ,        ^ 

trance   of  the   nerves.      1,    lactile  cor-  \  »      /            jt 

puscle    consisting  of  four  tactile  cells;    pUSCles. These,  SOHie- 

2,  twin  tactile  cells,  ts ;  a,  tactile  disc  ;  n,  timeg  ^m^  ^  corpuscks 

(to  the  left)    nerve  filament ;  n,  (to  the  * 

right)  medullated  nerve  fibre;    c,   true  of  Grandry,  or  the  COTpllS- 

skin.     (Stohr.)  des  0jr  Mgrke^   are  formed 

of  two    or   more    flattened    cells,    each    cell   being   about 

ttVo  °f  an  mc^  m  lengtn  by  -5^0"  °f  an  inc^  *n  breadth. 
A  medullated  nerve  fibre,  on  approaching  the  corpuscle, 
first  loses  the  white  substance  of  Schwann,  and  then  the 
axis-cylinder  ends  in  a  flat  disc  placed  between  two  of  the 
tactile  cells.  This  comparatively  simple  form  of  touch  cor- 
puscle is  found  in  the  skin  of  the  bills,  and  in  the  tongues, 
of  birds,  especially  those  of  aquatic  habits. 


The  Sense  of  ToucJl 


•47 


(c,  (3)  Compound  touch  corpuscles. — These,  termed  the 
corpuscles  of  Wagner,  or  the  corpuscles  of  Mezssner,  are  oval 
bodies,  from  ^-g  to  -^-^  of  an  inch  in  length,  and  ^-  to 


s~:i"o 


42TT 


of  an  inch  in  breadth,  found  in  the  papillae  of  the  true  skin, 
especially  in  the  palm  of  the  hand  and  sole  of  the  foot. 
The  number  of  these  bodies  is  very  considerable.  About 
fifty  ^n  each  square  millimetre  have  been  counted  on  the 
tip  of  the  forefinger.  A  like  area 
over  the  second  joint  contained 
twenty,  and  over  the  first  joint 
seven  or  eight.  About  fifteen  per 
square  millimetre  have  been  found 
in  the  skin  of  the  last  joint  of  the 
great  toe,  and  three  or  four  in  the 
like  area  of  the  sole  of  the  foot. 
Each  tactile  corpuscle  has  one  or 
two  medullated  nerve  fibres  twisted 
spirally  round  it  (Fig.  14),  and 
near  the  upper  pole  of  the  corpuscle 
the  white  substance  of  Schwann 
disappears,  and  the  axis -cylinder 
ends  in  little  excrescences  or  thick- 
enings. The  corpuscle  is  built  up 
of  flattened  cells,  the  edges  of 
which,  often  seen  in  section,  give 
it  a  peculiar  striated  appearance. 
These  bodies  are  evidently  con- 
structed on  the  same  plan  as  the 
more  simple  corpuscles  in  the  bills  of  birds,  above  described, 
each  consisting  of  a  number  of  tactile  cells. 

(d)  End-bulbs. — These,  sometimes  called  the  end-bulbs, 
or  e?id-knobs  of  Krause,  occur  in  the  conjunctiva  of  the  eye, 
the  mucous  membrane  of  the  mouth,  in  some  of  the  liga- 
ments of  joints,  occasionally  in  tendons,  and  in  the  sexual 


Fig.  14. — Touch  corpuscle,  a, 
Layers  of  connective  tissue  of 
the  true  skin  ;  b,  body  of 
corpuscle  ;  d,  d,  nerve  fibres 
twisted  spirally  round  the  cor- 
puscle ;  c,  nerve  fibres  at  the 
lower  end  of  the  corpuscle  ; 
e,  nerve  fibre  ending  in  little 
thickenings  near  the  upper  end 
of  the  corpuscle.  Magnified 
50  diameters. 


48 


Physiology  of  the  Senses 


organs.  They  have  also  been  found  on  the  under  surface 
of  the  toes  of  the  guinea-pig,  in  the  ear  and  body  of 
the  mouse,  and  in  the  wing  of  the  bat.  Varying  from 
3iro  t0  TTo  °f  an  ^nc^  *n  length,  each  consists  of  a  deli- 
cate wall  of  connective  tissue,  sometimes  forming  a  little 
sac,  in  the  interior  of  which  we  find  granular  matter  and 
nuclei  (Fig.  16).  Sometimes,  also,  the  granular  matter 
takes  the  form  of  a  knob.  The  nerve  may  apparently  end 
at  the  lower  extremity  of  the  bulb  (Fig.  15,  2),  or  it  may 


Fig.  15. — Various  forms  of  end- 
bulbs.     (Krause.) 


Fig.  16. — End -bulb,      a,  nerve;  b, 
connective  tissue  wall,    (Krause.) 


penetrate  it  and  form  a  number  of  loops  (Fig.  15,  1),  or 
it  may  end  in  a  long  ribbon  or  rod  (Fig.  15,  3). 

(e)  Pacinian  bodies. — These,  sometimes  termed  the  cor- 
puscles of  Pacini,  or  the  corpuscles  of  Vater,  from  -^~  to 
^  of  an  inch  in  breadth,  and  from  -—  to  \  of  an  inch  in 
length,  are  found  in  the  subcutaneous  connective  tissue  of 
the  palm  of  the  hand  (including  the  fingers)  and  sole  of  the 
foot,  in  the  sexual  organs,  in  the  deeper  layers  of  connect- 
ive tissue  below  the  skin  near  joints,  in  the  mesentery  (the 
fold  of  peritoneum  holding  the  intestine  in  position),  and  in 


The  Sense  of  Touch 


49 


IR" 


the  connective  tissue  around  the  pancreas.     They  have  also 
been   found  in  the  skin  of  the   elephant   and  of  the  bat. 
About  600  exist  on  the  palmar  surface 
of  each  hand,  and  as  many  on  each  foot. 
Each  Pacinian  corpuscle  consists  of  from 
forty  to  fifty  lamellae  or  capsules  (Fig. 
18)  concentrically  arranged.    The  space 
between  each  pair  of  lamellae  is  lined  by 
a  layer  of  flattened  cells,  and  is  filled 
with  fluid.      Each  capsule  is  smaller  as 
we  approach  the  centre,  and  the  capsules 
are  all  connected  at  the  pole  opposite 
the  entrance  of  the  nerve  by  a  thicken- 
ing.  A  small  artery  enters  the  corpuscle. 
The   nerve   of   the    medullated    variety 
enters  the  corpuscle  at  one  pole,  and  may 
be  regarded  as  forming  its  stem  or  stalk. 
The  fibre  perforates  the  capsules,  and  the 
axis-cylinder  runs  up  into  the  clear  mass 
in  the  centre  of  the  corpuscle,  the  medul- 
lary sheath  and  the  white  substance  of 
Schwann  terminating  at  the  entrance  of 
the  filament  into  the  clear  mass.      Near 
the  farthest  end  the  axis-cylinder  often 
divides  into  two  or  more  branches,  and    Fig.  17.— Diagrammatic 
these,  in  turn,  end  in  a  little  pear-shaped 
mass,   called  the  terminal  Hid.      Each 
bud  is  composed  of  a  dense  network  of 
minute   fibrils.      Surrounding  the   axis- 
cylinder  we  find  a  transparent  or  slightly 
striated  substance,  with  sometimes  rows 
of  nuclei,  especially  near  the  farther  end. 
Smaller   and   simpler  bodies,   but   con- 
structed on  the  same  plan,  have  been  found  in  the  bills  and 

E 


view  of  the  under  sur- 
face of  the  index  finger 
with  Pacinian  corpus- 
cles. «,  Nerve  ;  b,  c, 
lateral  and  terminal 
branches  of  the  nerves  ; 
d,  d,  d,  Pacinian  cor- 
puscles ;  1  first,  2 
second,  and  3  third 
phalanx  of  the  finger. 
(Schwalbe.) 


5° 


Physiology  of  the  Senses 


tongues  of  birds  (distinct  from  Grandry's  corpuscles),  and 

are  termed  the  corpuscles  of  Herbst. 

(3)  Nerve- endings  in  connection  with  tactile  hairs. — 

A  hair  grows  from  a  follicle  or  pit  in  the  substance  of  the 

true  skin.  A  layer  of  epidermis 
dips  down  into  the  follicle,  lining 
it,  and  covering  a  papilla  in  the 
bottom  of  the  follicle.  From 
the  surface  of  the  papilla,  which 
is  in  reality  one  of  the  papillae 
of  the  true  skin,  the  hair  is 
developed,  and  as  it  passes  up 
to  the  mouth  of  the  follicle,  it 
is  covered  by  a  sheath,  com- 
posed of  layers  similar  to  those 
of  the  epidermis.  Each  papilla 
on  which  a  hair  grows  is  richly 
supplied  with  capillary  blood- 
vessels. The  papillae  of  the 
special  tactile  hairs,  like  those 
near  the  mouth  of  a  cat,  are 
larger  and  more  vascular  than 
those  of  ordinary  hairs.  It 
would  appear  that  each  ordinary 
hair  follicle  is  supplied  with  fine 
nerves.  Fine  medullated  nerve 
fibres  form  a  network  in  the  outer 
coat  of  the  hair  follicle,  and  they 
then  lose  the  white  substance  of 
Schwann,  and  run  more  in  a 
longitudinal  direction,  parallel  to 
the  hair.     They  then  penetrate 

the  wall  of  the  follicle  and  end  in  the  inner  layer  of  the  sheath 

of  the  hair,  but  their  exact  mode  of  termination  is  yet  un- 


FlG.  18. — A  Pacinian  corpuscle.  N, 
nerve  ;  V,  V,  vessel ;  T,  nerve- 
ending.    (Klein  and  Noble  Smith.) 


The  Sense  of  Touch  51 

known.  The  number  of  nerve  filaments  brought  into  close 
relation  with  a  true  tactile  hair  is  very  great,  dense  net- 
works being  formed  both  in  the  inner  and  the  outer  sheaths 
of  the  hair,  and  they  end  in  small  knob -like  swellings  be- 
tween the  columnar  cells  of  the  outer  sheath  of  the  hair.  In 
some  cases  a  special  plexus  of  minute  nerve  fibrils  has  been 
found  surrounding,  like  a  ring,  the  neck  of  the  hair  follicle. 

It  is  well  known  that  tactile  hairs  can  be  voluntarily 
caused  to  stand  out  stiff  and  rigid.  This  is  owing  to  the 
fact  that  such  hairs  possess  a  special  arrangement  for  so 
erecting  the  hairs.  Surrounding  the  neck  of  the  hair  follicle 
we  find  sinuses  and  spaces  of  erectile  tissue,  controlled  by 
bands  of  elastic  and  unstriped  muscular  fibre.  When  the 
spaces  are  full  of  blood  the  hair  projects  from  the  centre  of 
a  highly  elastic  cushion,  thus,  no  doubt,  giving  greater  sen- 
sitiveness to  the  apparatus. 

The  small  woolly  hairs  on  the  skin  of  many  animals 
appear  to  be  organs  of  touch,  and  experiment  has  shown  that 
they  are  more  sensitive  than  the  areas  of  skin  between 
them.  In  many  animals  the  proper  tactile  hairs  acquire 
great  length,  thickness,  and  stiffness.  These  vibrissa^  or 
whiskers  or  mustaches,  in  marine  carnivora,  plunging  into 
depths  of  the  sea  where  there  is  little  or  no  light,  serve, 
according  to  Owen,  "  as  a  staff,  in  a  way  analogous  to 
that  held  and  applied  by  the  hand  of  a  blind  man." 
The  night -prowling  felines  and  the  nocturnal  monkeys, 
like  the  aye -aye,  have  hairs  of  this  kind  developed  on 
the  eyebrows,  lips,  and  cheeks.  Other  epidermic  append- 
ages serve  useful  purposes  in  connection  with  the  sense 
of  touch.  The  broad  massive  hoof  of  the  horse  is  not 
adapted  for  delicate  discriminations  of  tactile  sensations, 
but  clothing,  as  it  does,  highly  vascular  and  sensitive 
lamellae,  gives  broad  and  massive  sensations,  which 
enable    the    animals    to    appreciate    the    solidity   of   the 


5  2  Physiology  of  the  Senses 

ground  on  which  they  tread.  Animals  living  in  the  sea 
sometimes  have  touch  organs  developed  which  enable 
them  to  detect  pressures  or  movements,  often  at  a  con- 
siderable distance  from  them.  Thus  whales  have  large 
papillae  in  the  skin,  richly  supplied  with  nerves,  and  some- 
times the  skin,  bearing  these  papillae,  is  thrown  into  plaits 
or  folds,  so  as  to  give  a  greater  extent  of  sensitive  surface. 
It  is  said  by  Owen  that  this  arrangement  "is  peculiar 
to  the  swifter  swimming  whales  that  pursue  mackerel 
and  herring,  and  may  serve* to  warn  them  of  shoals, 
by  appreciation  of  an  impulse  of  the  water  rebounding 
therefrom,  and  so  conveying  a  sense  of  the  propinquity 
of  sunken  rocks  or  sand-banks."  The  nose-leaves  and 
sensitive  ears  of  some  of  the  bats  often  show  vibratile 
movements, — trembling,  like  the  antennae  of  insects,  as  the 
animal  gathers  information  as  to  its  environment, — and  thus 
act  as  delicate  organs  of  touch.  The  nose  and  feet  of 
burrowers  in  the  earth,  like  the  mole,  have  always  delicate 
organs  of  touch,  by  which  the  animals  feel  their  way  in 
their  subterranean  galleries. 

Nature  of  the  Tactile  Mechanism. — Touch  is  a  sensa- 
tion of  pressure  referred  to  the  surface  of  the  body.  When 
we  touch  anything  there  is  always  a  certain  amount  of 
pressure  between  the  sensitive  surface  and  the  body 
touched.  What  we  call  contact  is  gentle  pressure ;  a 
greater  amount  of  force  or  pressure  makes  the  sensation 
of  touch  more  acute  ;  by  and  by,  there  is  a  feeling  of 
resistance  to  pressure,  still  referred  to  the  skin  ;  when  a 
weight  is  supported  on  the  palm  of  the  hand  there  is  then  a 
sensation  of  muscular  resistance,  a  sensation  referred  not 
only  to  the  skin,  but  also  to  the  muscles,  and  by  which  we 
become  aware  that  the  muscles  are  contracted ;  and, 
finally,  the  pressure  may  be  so  great  as  to  cause  a  sensa- 
tion of  pain  which,  however,  may  be  confused  with  simul- 


The  Sense  of  Touch  53 

taneous  sensations  of  contact  and  of  muscular  resistance. 
The  force,  however,  that  gives  rise  to  touch,  in  its  various 
degrees,  may  not  act  vertically  on  a  sensory  surface,  but  in 
the  opposite  direction,  as  when  we  pull  a  hair.  Touch  is, 
therefore,  in  its  essence,  the  appreciation  of  mechanical 
force,  and  in  this  way  it  presents  a  strong  resemblance  to 
hearing,  which  is  a  more  delicate  kind  of  touch,  being  due 
to  variations  of  pressure  on  the  auditory  organ.  In  addi- 
tion, however,  to  sensations  of  touch,  contact  with  a  foreign 
body  may  give  rise  to  sensations  of  heat  or  cold — that  is 
to  say,  to  sensations  of  temperature.  Thus  when  we  place 
something  on  the  palm  of  the  hand,  the  resulting  sensation 
may  be  of  a  complex  character,  involving  sensations  of 
gentle  pressure  (contact),  of  more  severe  pressure,  and  of 
temperature.  True  tactile  impressions  are  absent  from 
internal  mucous  and  serous  surfaces,  as  has  been  proved  in 
men  having  fistulous  openings  into  the  stomach,  intestine, 
bladder,  or  pleural  cavities.  In  such  cases  pressure  does 
not  cause  a  sensation  of  touch,  but  of  pain. 

A  consideration  of  the  structure  of  the  terminal  organs 
of  touch,  above  described,  shows  that  they  must  serve 
(1)  for  protecting  the  extremities  of  the  sensory  nerves 
from  direct  pressure  ;  (2)  for  communicating  slight  varia- 
tions of  pressure  to  the  nerve-ending;  and  (3)  for  so 
modifying  external  pressures  as  to  give  them  more  or  less 
of  a  rhythmic  character.  Thus  if  we  consider  the  nerve- 
ending  in  an  end-bulb,  or  in  a  Pacinian  corpuscle,  lying 
in  a  fluid  or  semi-fluid  substance,  surrounded  by  one  or 
more  envelopes  of  connective  tissue,  we  see  that  most 
delicate  pressures  must  be  communicated  to  it,  and  also 
that  a  wave -like  movement  may  be  set  up  in  the  fluid 
matter,  thus  subjecting  the  nerve-ending  to  a  number  of 
intermittent  pressures  or  vibrations.  In  the  case  of  the 
touch  corpuscles,  either  simple  or  compound,  the  arrange- 


54 


Physiology  of  the  Senses 


ment  is  evidently  that  of  an  elastic  cushion  against  which 
the  nerve  filament  is  pressed,  thus  again  making  variable 
pressures  or  vibrations  possible.  In  like  manner,  move- 
ments communicated  to  a  hair,  the  follicle  of  which  is  sur- 
rounded by  elastic  structures  and  nerve-endings,  must  give 
rise  to  impulses  in  these  nerves,  probably  of  an  intermittent 
or  vibratory  character.  No  part  of  the  body,  when  touching 
anything,  can  be  regarded  as  absolutely  motionless,  and 
the  slight  oscillations  of  the  sensory 
surface,  and,  in  many  cases,  of  the 
body  touched,  produce  those  varia- 
tions of  pressure  on  which  touch  de- 
pends. 

Sensitiveness  of  the  Skin. — It  is 
a  familiar  observation  that  all  parts  of 
the  skin  are  not  equally  sensitive.  The 
method  of  determining  the  degree  of 
sensitiveness,  first  employed  by  Weber, 
consists  in  finding  the  smallest  dis- 
tance at  which  the  two  points  of  a  pair 
of  compasses  can  be  felt.  Two  in- 
struments suitable  for  such  observa- 
tions are  shown  in  Figs.  19  and  20,  and  the  results  in 
millimetres  l  are  given  in  the  following  table  : — 


Fig.  iq-—  Compasses  of 
Weber. 


Tip  of  tongue . 

I.I 

Centre  of  palm 

8-9 

Under    surface    of    third 

Under    surface    of    third 

phalanx  of  finger . 

2-2.3 

phalanx  of  great  toe 

11. 3 

Red  part  of  the  lip  . 

4-5 

Upper  surface  of  second 

Under  surface  of  second 

phalanx"  of  finger 

11. 3 

phalanx  of  finger. 

4-4-5 

Back       .... 

11. 3 

Upper    surface    of    third 

Eyelid     .... 

11. 3 

phalanx  of  finger 

6.8 

Under    surface   of    lower 

Tip  of  nose 

6.8 

third  of  forearm  . 

15.0 

ball  of  thumb 

6.57 

Cheek     .... 

15.8 

1   1  millimetre  =  -^'5-  of  an  inch. 


The  Sense  of  Touch 


55 


Temples 
Forehead 
Back  of  head 
Back  of  hand 
Knee 
Gluteal  region 


22.6  Forearm  and  leg      .         .45.1 

22.6  Neck       .         .         .         .54-1 

27. 1  Back,  opposite  fifth  dorsal 

31.6  vertebra       .         .         .54.1 

36. 1  Upper  arm,  thigh,  centre 

44.6.  of  back         .         .         .67.1 


Numerous  investigations  made  since  the  time  of  Weber 
have,  shown  considerable  variations  in  different  individuals. 
The  method  is  employed  by  physicians  in  the  diagnosis  of 
nervous  diseases  affecting  the  sensitiveness  of  the  skin. 
The  general  result  of  Weber's  method  is  to  show  that 
in  a  healthy  person  those*  parts  are  most  sensitive  as 
regards  the  power  of  discriminating  two  points  at  a  certain 


Fig.  20. — ./Esthesiometer  of  Sieveking. 

distance  from  each  other,  which  we  use  habitually  as  organs 
of  touch.  Thus  the  tips  of  the  fingers  on  their  under 
surface,  the  palms  of  the  hands,  the  margins  of  the  lips,  are 
more  sensitive  than  the  dorsal  surfaces  of  the  limbs  or  the 
skin  covering  the  back.  Sensitiveness  is  great  in  parts  of 
the  body  that  are  habitually  moved,  and  it  increases  from 
the  joints  towards  the  extremities.  Again,  sensitiveness  is 
finer  if  we  proceed  a  given  distance  along  the  transverse 
axis  of  a  limb  than  if  we  pass  the  same  distance  along  the 
long  axis. 

Moistening  the  skin,  stretching  it,  or  taking  baths  in  water 
containing  common  salt  or  carbonic  acid,  increases  sensi- 
tiveness, especially  as  regards  the  power  of  discriminating 
points.      An   anaemic  condition,   venous   congestion,   cold, 


56  Physiology  of  the  Senses 

and  the  use  of  solutions  of  atropine,  daturine,  morphine, 
strychnine,  alcohol,  bromide  of  potassium,  cannabine,  and 
hydrate  of  chloral,  blunt  sensibility.  Moistening  the  skin 
with  a  solution  of  caffeine  is  said  to  increase  sensibility. 

Sense  of  Locality. — Not  only  is  the  skin  sensitive,  but 
one  is  able,  with  great  precision,  to  determine  the  part 
that  has  been  touched.  This  power  may  be  termed  the 
sense  of  locality.  The  general  law  is  that  the  greater  the 
number  of  sensory  nerves  in  a  given  area  of  skin,  the  greater 
is  the  degree  of  accuracy  in  distinguishing  different  points, 
and  in  determining  locality.  Contrast,  for  example,  the  tip 
of  the  finger  with  the  back  of  the  hand. 

One  would  imagine  that  the  habitual  use  of  these  parts 
would  so  educate  the  mind  as  to  enable  us  to  identify 
particular  parts  touched,  even  although  these  parts  might 
not  be  much  more  sensitive  than  other  parts.  It  is  doubt- 
ful, however,  if  exercise  improves  sensitiveness.  Thus 
Galton  found  that  the  performances  of  blind  boys,  when 
examined  by  the  Weberian  method,  were  not  superior  to 
those  of  other  boys,  and  he  says  "  that  the  guidance  of  the 
blind  mainly  depends  on  the  multitude  of  collateral  indi- 
cations, to  which  they  give  much  heed,  and  not  their 
superiority  in  any  of  them." 

Absolute  Sensitiveness.- — Hitherto  we  have  been  dis- 
cussing the  power  of  discriminating  points,  but  this  is 
different  from  the  absolute  sensitiveness  of  any  part  of  the 
skin.  What  is  the  smallest  pressure  that  can  give  rise  to  a 
sensation,  and  what  is  the  smallest  difference  that  can  be 
observed  between  two  sensations  ?  Many  attempts  have 
been  made  to  answer  these  questions.  Thus  small  weights 
have  been  allowed  to  press  on  the  skin,  and  the  smallest 
weight  causing  a  sensation,  and  the  smallest  difference 
between  two  weights,  have  been  noted.  Again,  an  ordinary 
balance   has   been   used,   and  from    the   under    surface   of 


The  Sense  of  Touch  57 

one  scale-pan  a  fine  needle  projected  which  pressed  on  the 
skin,  while  weights  were  placed  in  one  scale -pan  or  the 
other  according  to  the  nature  of  the  experiment.  In  this 
way  accurate  measurements  were  obtained.  To  avoid  the 
interfering  effects  of  sudden  shocks,  the  skin  has  been 
pressed  against  a  fine  tube  containing  water,  so  that  rhythmic 
waves,  like  those  of  the  pulse,  were  caused  to  impinge  on  the 
skin.  The  general  results  of  these  methods  may  be  briefly 
summarised  thus  : — 

(1)  The  greatest  acuteness  was  observed  on  the  fore- 
head, temples,  and  back  of  the  hand  and  forearm,  which 
detected  a  pressure  of  .002  gramme.1  The  skin  of  the 
fingers  detected  .005  to  .015  gramme,  and  the  chin,  abdo- 
men, and  nose  .04  to  .05  gramme. 

(2)  One  gramme  was  placed  on  the  skin,  and  the 
least  additional  weight,  in  grammes  or  fractions  of  a 
gramme,  that  could  be  appreciated  was  then  determined, 
with  the  following  result  : — Third  phalanx  of  finger, 
.499;  back  of  the  foot,  .5  ;  second  phalanx,  .771  ;  first 
phalanx,  .82  ;  leg,  1  ;  back  of  hand,  1.156  ;  palm,  1.108  ; 
patella,  1.5  ;  forearm,  1.99;  umbilicus,  3.5;  back,  3.8. 
The  greatest  absolute  sensitiveness  to  a  single  pressure 
was  on  the  back  of  the  hand,  while  the  greatest  power  of 
discriminating  differences  of  pressure  (and  also  of  discrim- 
inating points)  was  on  the  palmar  surface.  Eulenberg  puts 
the  matter  thus  :  the  skin  of  the  forehead,  lips,  cheeks,  and 
temples  appreciated  differences  of  pressure  to  the  extent 
of  from  -^  to  -i.  of  the  first  pressure ;  the  back  of  the 
last  phalanx  of  the  fingers,  the  forearm,  hand,  first  and 
second  phalanges,  the  palmar  surface  of  the  hand,  fore- 
arm, and  upper  arm,  distinguished  differences  of  -i-  to  ^  ; 
and  then  follow  the  back  of  the  foot  and  toes,  the  sole 
of  the  foot,    and   the   back  of   the  leg  and   thigh,    all   of 

1  A  gramme  =  15.432  grains. 


58  Physiology  of  the  Senses 

which  require  a  greater  difference  than  -~  of  the  original 
weight. 

(3)  In  passing  from  light  to  heavier  weights,  the  acute- 
ness  at  once  increases,  a  maximum  is  reached,  and  then, 
with  still  heavier  weights,  the  power  of  distinguishing  differ- 
ences gradually  diminishes  and  finally  disappears. 

Fusion  of  Tactile  Impressions. — If  the  finger  is  held 
against  a  blunt  toothed  wheel,  and  the  wheel  is  rapidly 
rotated,  a  smooth  margin  is  felt  when  the  intervals  of  time 
between  the  contacts  of  successive  teeth  are  less  than  T^ 
to  -q^  of  a  second.  The  same  experiment  may  be  made 
by  pressing  the  finger  gently  over  the  holes  in  one  of  the 
outermost  circles  of  a  large  syren  rotating  quickly;  the  sensa- 
tion of  touching  individual  holes  disappears,  and  there  is  a 
feeling  of  touching  a  slit.  The  meaning  of  these  experi- 
ments is  that  the  individual  impressions,  if  they  follow  each 
other  with  sufficient  rapidity,  are  fused  together  in  conscious- 
ness, so  that  we  experience  one  continuous  sensation.  By 
attaching  light  bristles  to  the  prongs  of  rapidly  vibrating 
tuning-forks,  and  bringing  the  bristles  into  gentle  contact 
with  the  tips  of  the  fingers,  and  especially  with  the  margins 
of  the  lips,  curious  observations  may  be  made.  If  the 
forks  are  vibrating  at  rates  of  from  600  to  1500  vibrations 
per  second,  sensations  of  an  acute  and  almost  unbearable 
character  are  experienced,  but  above  this  limit,  sensation, 
other  than  that  of  mere  contact,  almost  or  wholly  disappears, 
although  the  fork  is  in  active  vibration. 

After- tactile  Impressions. — If  the  weight  be  consider- 
able, and  if  it  be  allowed  to  press  on  the  skin  for  a  few 
minutes  and  be  then  removed,  a  faint  sensation  of  pressure 
may  continue  for  a  few  seconds.  This  is  termed  an  afte?- 
effect.  It  shows  that  the  influence  on  the  nerves  or  nerve 
centres  does  not  disappear  the  instant  the  exciting  cause  is 
removed.      Thus  we  may  compare  impressions,  and  thus 


The  Sense  of  Touch  59 

the  effect  of  one  impression  is  more  easily  fused  with  the 
effect  of  impressions  following  quickly  after  it. 

Information  from   Tactile    Impressions. — When  the 
skin  comes  into  contact  with  the  surface  of  any  external 
body,   we    become   aware    of   the    existence   of  something- 
touching  the  sensory  surface,  and  from  the  intensity  of  the 
sensation  we  form  a  judgment  as  to  the   intensity  of  the 
pressure.      As  already  pointed  out,  we,  in  the  first  instance, 
refer  the   sensation   to    the    skin,    but    after    the    pressure 
has  reached   a   certain  intensity,   so  as  to  call  forth  mus- 
cular action  to  resist  it,  the  sensation  of  touch  (pressure) 
is  commingled  with  that  of  the  so-called  muscular  sense. 
The  number  of  points  on  the  surface  of  the  foreign  body  that 
individually  touch  the  skin  enables  us  to  judge  of  its  smooth- 
ness or  roughness.     Thus,  if  uniformly  smooth  it  gives  rise 
to  a  sensation  like  that  of  touching  a  billiard  ball,  and  if 
we  move  the  hand  over  a  considerable  distance  of  smooth 
surface  there  is  a   sensation  of  massiveness,  as  when  we 
touch  a  marble  slab.      On  the  other  hand,  a  body  having 
points    irregular   in   size   and   number  in   a   given  area   is 
rough  ;  and  if  the  points  are  very  close  together,  like  that 
of  the  pile  of  velvet,  a  peculiar  sensation  of  roughness  may 
be    experienced,    almost   intolerable    to    some    individuals. 
If  a  large  area  of  skin  be  uniformly  pressed   upon,  the 
sensation  of  pressure  may  disappear  after  a  few  minutes, 
and  there  will  be  sensation  only  when  there  is  a  variation 
of  pressure.      Again,  if  one  part  of  the  body  is  subjected  to 
one  pressure,   and  an  adjacent  part   to   another  pressure, 
the    sensation   of  pressure    may    be    limited    to    the    line 
dividing  the  one  area  from  the  other.      Thus  if  we  plunge 
the   finger   into   a   dish  of  mercury,  a  ring  of  constriction 
may  be  felt  just  at  the  junction  of  the  surface  of  the  mer- 
cury with  the  air.     The  same  is  experienced  when  the  body 
is  immersed  in  a  bath.      No  feeling  of  pressure  is  felt  in 


60  Physiology  of  the  Senses 

the  immersed  parts,  but  if  the  arm  or  leg  be  lifted  into  the 
air,  a  sense  of  pressure  may  be  experienced  on  the  strip  of 
skin  where  the  limb  passes  from  the  water  into  the  air. 

The  tactile  field.  As  already  pointed  out,  we  can  deter- 
mine, with  great  accuracy,  the  part  touched,  and  from  this 
the  probable  position  of  the  touching  body.  If  a  point 
of  the  skin  is  touched  certain  tactile  corpuscles  are  irritated  ; 
these,  in  turn,  set  up  impulses  in  sensory  nerve  fibres,  and 
these  impulses  are  carried  by  the  fibre,  first  to  the  spinal 
cord,  and  then  to  the  brain,  where  the  fibres  end  in  gan- 
glionic masses  in  the  gray  matter  on  the  cerebral  cortex. 
There  are  thus,  projected,  as  it  were,  on  the  cortex  of  the 
brain,  tactile  centres  for  the  hind-leg,  fore-leg,  neck,  eye, 
ear,  trunk,  etc.,  and  it  follows  that  each  point  of  the  skin 
has  a  corresponding  point  in  the  cerebral  cortex.  Thus  for 
each  stimulation  of  a  point  of  the  cerebral  cortex  there  is  a 
local  sign,  and  thus  we  localise  tactile  impressions.  There  is 
thus  in  consciousness,  and  in  the  brain,  a  tactile  field,  to 
which  all  points  of  the  skin  surface  may  be  referred,  point 
for  point.  This  is  comparable  to  the  visual  field  to  which 
all  retinal  impressions  are  related,  and  which  will  be  after- 
wards discussed.  We  have,  as  it  were,  a  tactile  picture  of 
the  part  touched,  and  when  we  pass  the  hand  over  any 
external  object  (supposing  the  eyes  to  be  shut)  we  touch  it 
at  various  points,  and  from  the  differences  of  pressure,  and 
from  a  comparison  of  the  positions  of  the  various  points  in 
the  tactile  field,  we  judge  of  the  configuration  of  the  body. 
We  obtain  a  number  of  tactile  pictures,  and  these  are  fused 
together  so  as  to  give  a  conception  of  the  whole  object.  If 
the  object  be  large,  we  do  not  depend,  however,  on  tactile 
pictures  only.  It  may  be  necessary  to  move  the  limb,  or 
even  the  body  itself,  so  as  to  examine  the  whole  of  the 
external  object,  and  the  sensations  arising  from,  or  connected 
with,  the  muscular  movement  are,  in  turn,  fused  with  the 


The  Sense  of  Touch 


61 


tactile  pictures.  We  then  judge  of  the  form,  size,  and 
nature  of  surface  of  the  body  touched.  If  there  is  an 
abnormal  displacement  of  position  of  the  body  touched,  or 
if  we  touch  it  with  parts  of  the  body  that  we  are  not  in  the 
habit  of  using  for  this  purpose,  a  false  conception  may  arise 
as  to  the  shape  of  the  body.  Thus,  in  the  old  experiment 
of  Aristotle,  shown  in  Fig.  21,  if  a  pea  be  placed  between 
the  index  and  middle  finger,  so  as  to  touch  the  outer  side  of 
the  index  finger  and  the  inner  side  of  the  middle  finger, 
a  sensation  of  touching  one  round  body  is  experienced  ;  but 
if  the  fingers  be  crossed,  so  that  the  pea 
touches  the  inner  side  of  the  index  finger 
and  the  outer  side  of  the  middle  finger, 
there  will  be  a  sensation  of  two  round 
bodies,  because,  in  these  circumstances, 
there  is  added  to  the  feelings  of  contact 
a  feeling  of  distortion  (or  of  muscular 
action)  like  what  would  arise  if  the  fingers, 
for  purposes  of  touch,  were  placed  in 
that  unnatural  position. 

The  knowledge  of  the  tactile  field  is 
usually  precise  and  definite.  This  is  illus- 
trated by  the  well-known  fact  that  when    FlG-  21. -Experiment  of 

Aristotle. 

a  piece  of  skm  has  been  transplanted  from 
the  forehead  to  the  nose,  in  the  operation  for  removing  a  de- 
formity of  the  nose  caused  by  ulcerative  disease,  the  patient 
may  feel  the  new  nasal  part  as  if  it  were  in  his  forehead,  and 
he  may  have  a  headache  in  his  nose.  The  mind  receives  the 
messages  thus  transmitted  to  definite  points  in  the  cortex, 
and  assumes  that  these  messages  come  from  the  locality  from 
which  similar  messages  have  come  over  and  over  again. 
Thus  it  is  that  a  man  may  feel  pain  in  the  toes  of  an  am- 
putated limb ;  and  a  medical  man,  who  had  the  misfortune 
to  lose  his  leg  by  amputation,  told  the  writer  that  for  years 


62  Physiology  of  the  Senses 

he  sometimes  felt  pain  in  a  troublesome  corn  that  once 
existed  in  the  amputated  member.  There  can  be  no  doubt, 
however,  that  our  knowledge  of  the  tactile  field  depends 
largely  on  the  education  of  the  sense,  not  merely  in  the 
individual,  but  in  the  race.  Even  in  the  individual  much 
may  be  done  to  improve  it.  Few,  for  example,  have  any 
knowledge  of  touching  anything  with  the  third  toe,  because 
this  part  of  the  body  is  not  used  in  collecting  tactile  in- 
formation, but  a  little  practice  will  soon  show  any  one  that 
sensations  may  be  referred  to  this  part  with  almost  as  great 
ease  as  to  the  ball  of  the  great  toe,  which  is  in  habitual  use. 
Theories  as  to  Touch. — Various  theories  have  been  pro- 
pounded to  explain  the  phenomena  of  tactile  sensibility, 
but  it  cannot  be  said  that  any  one  is  wholly  satisfactory.  The 
oldest,  first  put  forth  by  Weber,  and  modified  by  various 
psychologists,  states  that  while  we  refer  every  tactile  sensa- 
tion to  a  certain  position  in  the  tactile  field,  we  do  not  refer 
it  merely  to  a  point,  but  to  a  minute  area  of  skin,  which  has 
been  termed  a  circle  of  sensibility.  It  is  also  assumed  that 
we  can  refer  a  sensation  to  each  circle,  as  when  we 
touch  the  skin  with  the  point  of  one  leg  of  the  compasses 
in  Weber's  experiment,  above  described.  If,  however,  we 
bring  both  points  within  one  circle,  we  still  have  a  sensation 
of  one  contact,  not  of  two  contacts.  Even  if  the  point  of 
the  second  leg  be  placed  on  another  circle  immediately 
adjoining,  there  is  still  a  sensation  of  only  one  contact,  and 
to  secure  a  sensation  of  two  contacts  it  is  necessary, 
according  to  this  theory,  to  have  always  one  or  more  circles 
intervening,  or,  to  put  the  matter  in  another  form,  there 
must  always  be  "a  non-irritated  sensory  element"  between 
the  two  points  touched.  It  is  also  supposed  that  each 
circle  has  its  own  nerve  fibre.  There  is  no  proof,  however, 
that  this  is  the  case.  The  extent  of  such  hypothetical 
circles  can  be  altered  by  practice  and  efforts  of  attention. 


The  Sense  of  Touch  63 

We  may  therefore  assume  either  that  the  circles  overlap,  or 
that  even  the  same  circle  may  be  innervated  by  several 
nerve  filaments,  so  that  when  any  part  of  the  circle  is 
touched,  various  nerve  filaments  may  be  excited.  One  can 
conceive,  however,  that  the  nerve  filaments  in  one  circle 
may  not  be  excited  to  an  equal  degree,  and  that  the  result- 
ing sensation  may  thus  be  variously  modified.  The  sug- 
gestion of  Krause,  that  the  sensitiveness  depends  on  the 
number  of  tactile  corpuscles  in  a  given  area,  is  worthy  of 
special  notice.  He  states  that  the  distance  of  the  two 
points  of  the  compasses  at  which  two  points  are  felt  in- 
cludes, in  the  mean,  twelve  tactile  corpuscles.  It  is  no 
doubt  true  that  tactile  corpuscles  are  not  absolutely  essential 
to  touch.  The  cornea  is  sensitive,  and  yet  it  contains  no 
such  bodies,  and  when  portions  of  the  skin  which,  by 
experiment,  were  found  sensitive  to  touch,  were  extirpated 
and  microscopically  examined,  no  touch  bodies  were  found. 
Still,  on  the  other  hand,  we  know  that  where  the  sense  of 
touch,  and  especially  the  power  of  discrimination  of  points, 
is  more  acute,  there  touch  corpuscles  abound  ;  so  we  are  en- 
titled to  conclude  that  they  act  as  accessory  mechanisms  to 
the  sense.  Further,  it  must  not  be  forgotten  that  processes 
occur  in  the  nerve  centres,  and  that  we  must  not  look  to 
the  skin  alone  for  an  anatomical  explanation.  When  a 
nerve  cell  in  the  brain  receives  a  nervous  impulse  by  a  nerve 
originating  in  a  given  area  of  skin,  the  impression  may  be 
diffused,  by  irradiation,  to  neighbouring  nerve  cells,  which 
are  connected  by  nerve  fibres  with  adjoining  areas  of  skin. 
If  this  be  so,  then  the  effect  on  these  cells — in  accordance 
with  the  law  that  sensations  in  nerve  centres  are  referred 
to  the  origins,  in  the  periphery,  of  the  sensory  nerve  fibres 
reaching  them — will  be  referred  to  the  adjoining  areas  of 
skin,  or,  in  other  words,  to  adjoining  points  in  the  tactile 
field. 


64  Physiology  of  the  Senses 

Sensations  of  Temperature. — The  skin  is  also  the 
organ  by  which  we  appreciate  temperature,  and  it  is  not 
improbable  that  there  are  thermal  nerves  and  thermal 
end-organs.  Sensations  of  heat  and  cold  can  only  be  felt 
by  the  skin.  Direct  irritation  of  a  nerve  does  not  give  rise 
to  these  sensations.  Thus  if  we  plunge  the  elbow  into  very 
hot  water,  or  into  ice-cold  water,  we  do  not  experience 
heat  or  cold  by  thus  irritating  the  ulnar  nerve,  which 
lies  here  just  below  the  skin,  but  there  is  a  painful  sen- 
sation referred  to  the  extremities  of  the  nerve.  The  ex- 
posed pulp  of  a  diseased  tooth,  when  irritated  by  hot 
or  cold  fluids,  gives  rise  to  pain,  not  to  sensations  of  tem- 
perature. Recent  obser- 
.•  .••:  •••o** «*  .••.„•  *  \'\\ •*•>">      vations  show  that  there 

:     •••  •  £••:•!. C:»  •••      are  minute  areas  on  the 

••  •  •*«••  •*•••*         • 

••""•jV'V  •••/:.'  •'..*•{•••••.••:.       skin  in  which  sensations 

»••    ••  /••J  •  •     • 

•     •&*    "•  ••"""•:  **^%    •".    of  heat  and  cold  may  be 

.  "v.  V,:  •••.•  "•."**•-"  "  c#^':  more  acutely felt  than  m 

'•  adjoining  areas.      Some 

of  these  areas  are  more 

hence  are  called  cold 
spots,  and  others,  more  sensitive  to  heat,  have  received 
the  name  of  hot  spots,  and  they  appear  to  be,  or  to  con- 
tain, end-organs,  arranged  in  points,  subservient  to  a 
temperature  sense.  A  topographical  view  of  such  spots 
on  the  radial  half  of  the  dorsal  surface  of  the  wrist, 
as  depicted  by  Goldscheider,  is  shown  in  Fig.  22.  A 
simple  method  of  demonstrating  this  curious  phenomenon 
is  to  use  a  solid  cylinder  of  copper,  eight  inches  in  length, 
by  J  inch  in  thickness,  and  sharpened  at  one  end  to  a  fine 
pencil-like  point.  Dip  the  pointed  end  into  hot  water, 
close  the  eyes  and  touch  parts  of  the  skin.  When  a  hot 
spot   is   touched  there  is   an   acute  sensation   of  burning. 


The  Sense  of  Touch  65 

Such  a  spot  is  often  near  a  hair.  Again,  in  another  set  of 
experiments,  dip  the  copper  pencil  into  ice-cold  water  and 
search  for  the  cold  spots.  When  one  of  these  is  touched, 
a  curious  sensation  of  cold,  as  if  gathered  to  a  point,  is 
experienced.  It  will  be  found,  in  this  way,  that  in  a  given 
area  of  skin  there  may  be  hot  spots,  cold  spots,  and  tactile 
spots.  Cold  spots  are  more  abundant  than  hot  spots. 
The  spots  are  arranged  in  curved  lines,  but  the  curve 
uniting  a  number  of  cold  spots  does  not  coincide  with  the 
curve  forming  a  chain  of  hot  spots.  Both  spots  may  be 
perceived  as  double,  by  the  Weberian  method,  but  we  can 
discriminate  cold  spots  at  a  shorter  distance  than  hot  spots. 
Thus  on  the  forehead  cold  spots  have  a  minimum  dis- 
tance of  .8  mm.  and  hot  spots  4  mm.  ;  on  the  skin  of  the 
breast,  cold  spots  2  mm.  and  hot  spots  5  mm.  ;  on  the 
back,  cold  spots  1.5  mm.  and  hot  spots  4  to  6  mm.  ;  on 
the  back  of  the  hand,  cold  spots  3  mm.  and  hot  spots 
4  mm.  ;  on  the  palm,  cold  spots  .8  mm.  and  hot  spots 
2  mm. ;  and  on  the  thigh  and  leg,  cold  spots  3  mm.  and 
hot  spots  3.5  mm.  No  terminal  organ  for  this  sense  has 
yet  been  found.  Electrical  and  mechanical  stimulation  of 
the  hot  or  cold  spots  call  forth  the  corresponding  sensa- 
tion. This  indicates  that  a  special  terminal  organ  probably 
exists. 

It  is  highly  probable  that  there  are  nerve  filaments 
specially  devoted  to  conveying  to  the  nerve  centres  what 
may  be  termed  thermal  impressions,  and  possibly  there 
may  be  parts  of  the  brain  specially  connected  with  the 
translation  of  such  impressions  into  sensations  of  tempera- 
ture. A  leg  sent  to  "sleep"  by  pressure  on  the  sciatic 
nerve  will  be  found  to  be  less  sensitive  to  heat,  but  dis- 
tinctly sensitive  to  cold.  In  some  cases  of  disease  it 
has  been  noticed  that  the  skin  is  sensitive  to  a  tem- 
perature above  that  of  the  limb,   but  insensitive  to   cold. 

F 


66  Physiology  of  the  Senses 

Tactile  and  thermal  sensations  affect  each  other.  Thus 
a  weight  is  always  felt  to  be  heavier  when  it  is  cold 
than  when  it  is  hot,  and  the  minimum  distance  at  which 
two  compass  points  are  felt  is  diminished  when  one 
point  is  warmer  than  the  other.  Not  unfrequently  in 
diseases  of  the  nervous  system  tactile  sensibility  may  be 
diminished  or  increased  without  the  sense  of  temperature 
being  affected,  and  the  reverse  condition  also  occurs. 

The  skin,  as  an  organ  for  the  appreciation  of  tempera- 
ture, may  be  considered  from  another  point  of  view.  In  a 
warm-blooded  animal  (that  is  an  animal  possessing  a  heat- 
regulating  mechanism  by  which  the  mean  temperature  of 
its  body  is  maintained  fairly  constant  although  the  tempera- 
ture of  the  surrounding  medium  may  vary  within  wide 
limits)  the  mean  temperature  of  the  skin  is  regulated  by  the 
amount  of  blood  passing  through  it  in  a  given  time,  and  by 
the  degree  of  activity  of  the  sweat  glands.  Heat  is  lost 
from  the  skin  both  by  radiation  and  conduction.  If  a  man 
stands  before  a  thermal  pile  connected  with  a  sensitive 
galvanometer,  the  radiant  heat  from  his  body  is  at  once 
detected  by  the  movement  of  the  needle  of  the  galvano- 
meter. In  this  case  heat  leaves  his  body  by  radiation,  and 
also  reaches  the  thermal  pile  by  convection  through  the  air. 
Again,  when  he  stands  before  a  fire  he  becomes  warm, 
heat  entering  the  body.  When  he  touches  anything  it  feels 
hot  or  cold,  according  as  it  conducts  heat  out  of  or  into  the 
skin.  In  this  way  the  amount  of  heat  entering  or  issuing 
from  the  skin  is  constantly  varying,  and  the  skin  appreciates 
these  variations.  When  any  part  of  the  skin  is  above  its 
normal  mean  temperature,  warmth  is  felt ;  in  the  opposite 
case,  cold.  The  following  are  the  chief  points  that  have 
been  ascertained  regarding  the  appreciation  of  variations  of 
temperature. 

(i)  With  a  skin  temperature  of  from  150. 5  C.  to  35°  C, 


The  Sense  of  Touch  67 

the  tips  of  the  fingers  can  distinguish  a  difference  of  .2°  C. 
Temperatures  below  that  of  the  blood  (330  C.  to  270  C.) 
are  distinguished  by  the  more  sensitive  parts  even  to 
.05°  C. 

(2)  Parts  having  the  thermal  sense  acute  occur  in  the 
following  order :  Tip  of  tongue,  eyelids,  cheeks,  lips,  neck, 
belly.  The  smallest  difference  of  temperature,  in  degrees 
centigrade,  appreciated  by  the  skin  of  the  breast  is  .4° ;  back, 
.9° ;  back  of  hand,  .3° ;  palm,  .4°;  arm,  .2° ;  back  of  foot,  .4°; 
thigh,  .5°;  leg,  .6°  to  .2° ;  cheek,  .4°;  temple,  .3°,  giving 
a  mean  of  about  .3° — that  is,  t3q-  of  a  degree  centigrade. 

(3)  Sensations  of  heat  and  cold  may  alternate.  Thus, 
if  we  dip  the  hands  into  water  at  1  o°  C.  we  feel  cold  ;  then 
transfer  them  to  water  the  temperature  of  which  is  160  C. 
and  there  will  be  a  feeling  first  of  warmth  and  then  of  cold. 

(4)  The  extent  of  the  area  subjected  to  heat  or  cold 
influences  the  sensation.  For  example,  the  whole  hand 
dipped  into  water  at  29°.5  C.  feels  warmer  than  when  the 
finger  is  dipped  into  water  having  a  temperature  of  320  C. 

(5)  Great  sensibility  to  differences  of  temperature  is 
noticed  after  removal,  alteration  by  vesicants,  like  can- 
tharides,  mustard,  or  strong  acetic  acid,  or  destruction  of 
the  epidermis,  and  in  the  skin  affection  (known  to  be  of 
nervous  origin)  termed  herpes  zoster.  On  the  other  hand, 
removal  of  the  epidermis  increases  tactile  sensibility. 

Pain. — The  sensation  termed  pain  is  often  referred  to 
the  skin,  and  is  due  to  direct  irritation  of  sensory  nerves. 
Ordinary  sensory  nerves  convey  impressions  from  all  parts 
of  the  body  to  the  nerve  centres,  and  these  impressions  give 
rise  to  sensations,  often  of  a  vague  and  evanescent  charac- 
ter, such  as  a  feeling  of  general  bodily  comfort,  free  or 
obstructed  breathing,  hunger,  thirst,  fatigue,  etc.  If  such 
nerves  are  more  strongly  irritated  the  sensation  becomes  one 
of  pain,  and,  in  accordance  with  the  law  of  the  peripheral 


68  Physiology  of  the  Senses 

reference  of  sensation,  the  sensation  may  be  referred  to  the 
origin  of  the  nerve  in  the  skin.  Sometimes  this  pain  is 
distinctly  located,  but  in  other  cases  it  may  be  irradiated 
in  the  nerve  centres,  and  then  referred  to  areas  of  skin  or 
regions  of  the  body  which  are  not  really  the  seat  of  the 
irritation.  The  acuteness  or  intensity  of  pain  depends 
partly  on  the  intensity  of  the  irritation,  and  partly  on  the 
degree  of  excitability  of  the  sensory  nerves  at  the  time. 
Sometimes,  for  example,  the  excitability  of  sensory  nerves 
may  be  so  high  that  a  whiff  of  air  may  cause  acute  distress. 
If  only  a  few  nerves  are  affected  the  pain  is  acute  and 
piercing,  but  if  many  nerves  are  involved  it  may  be  more 
massive  and  diffuse  in  character.  The  quality  of  pain — 
whether  it  is  piercing,  cutting,  throbbing,  gnawing,  dull,  or 
boring — depends  on  the  nature  of  the  irritation,  and  on 
whether  the  irritation  is  constant  or  intermittent.  Lastly,  in 
many  nervous  diseases  involving  the  centres  of  sensation, 
disordered  sensations  may  be  referred  to  the  skin,  such  as 
abnormal  feelings  of  heat  or  cold,  creeping,  itching,  burn- 
ing, or  a  sensation  of  insects  crawling  in  the  skin,  all  giving 
rise  to  great  distress. 

The  Muscular  Sense. — As  a  rule,  we  do  not  judge  of 
the  weight  of  a  body  by  the  sense  of  pressure  on  the  skin 
alone,  but  we  lift  the  body  and  come  to  a  conclusion  as  to 
its  weight  by  a  sense  of  the  muscular  tension  necessary 
to  support  it  against  gravity.  This  is  the  so-called  mus- 
cular sense.  It  depends  on  sensory  nerves  originating  in 
the  muscles,  and  carrying  impressions  from  these  to  the 
nerve  centres.  Weber  made  some  ingenious  experiments 
on  the  delicacy  of  the  muscular  sense.  Thus  he  placed 
certain  weights  in  a  cloth,  and  held  it  suspended  by  the  four 
corners,  so  as  thus  to  remove  the  effect  of  pressure  or  fric- 
tion, and  then  he  endeavoured  to  form  a  judgment  as  to 


The  Sense  of  Touch  69 

the  weights  by  the  sensations  of  muscular  resistance  referred 
to   the   muscles   of  the  forearm.      He  found  that   he   was 
unable  to  form  a  correct  estimate  of  the  amount  of  the 
weight  either  by  the  muscular  sense  or  by  the  tactile  sense, 
but  he  found  the  muscular  sense  more  discriminating  than 
the  tactile  sense  as  to  estimation  of  differences  of  weight. 
Thus,  by  the  muscular  sense  he  was   able  to  distinguish 
weights  the  ratio  of  which  was  as   39  :  40,  while   by   the 
tactile  sense  (sense  of  pressure)  he  could  only  distinguish 
weights  the  ratio  of  which  was  as  29  :  30.     There  is  not 
so   accurate   a   perception   of   locality  in   connection   with 
muscular  as  there  is  in  the  case  of  tactile  impressions — that 
is  to  say,  there  is  no  well-defined  muscular  field  like  the 
tactile   field.      In   actual   experience,  tactile  and   muscular 
impressions  are  blended  so  as  to  give  a  sharp  representa- 
tion of  the  position  at  any  time  of  the  parts  of  the  body,  as 
well  as  of  any  change  in  such  position  brought  about  even 
by  a  passive  movement.     Thus,  if  we  place  the  arm  of  a 
blindfolded  person  across  the  chest,  he  is  immediately  con- 
scious of  the  position  of  the  limb,  although  he  has  made  no 
muscular  effort.     Finally,  when  active  movements  are  made 
by  which  the  limb  is  placed  in  a  certain  position  in  space, 
we  have  contributing  to  the  mental  representation  of  this 
position,  not  only  tactile  and  sensory  muscular  impressions, 
but  also  the  sense  of  effort  necessary  to  cause  the  muscles 
actively  to  perform  the  requisite  movement.     This  sense  of 
effort  may  be  called  a  sense  of  innervation,  and  is  distinct 
both  from  the  muscular  sense,  properly  so  called,  and  from 
the  tactile  sense. 


THE  SENSE  OF  TASTE 

This  sense  is  located  chiefly  in  the  tongue,  but  sensations 
of  taste  may  also  be  referred  to  the  soft  palate  and  even 
to  the  region  of  the  fauces.  The  tongue  is  a  muscular 
organ  covered  with  mucous  membrane.  By  means  of  its 
complicated  movements  it  plays  an  important  part  in 
chewing,  in  swallowing,  and  in  articulate  speech.  The 
mucous  surface  of  the  organ  is  covered  with  minute 
prominences  or  papilla^  of  which  there  are  three  kinds. 
Most  abundant  are  the  filiform  papilla,  small  cylindrical 
bodies,  about  one- twelfth  of  an  inch  in  length.  Inter- 
spersed with  these  are  the  fungiform  papillce,  so  called 
because  each  consists  of  a  narrow  stem  supporting  a 
flattened  top,  something  like  the  shape  of  a  mushroom. 
They  are  shorter  than  the  filiform  papillae,  varying  from 
one-fiftieth  to  one-twelfth  of  an  inch  in  length,  and  they  may 
often  be  detected  by  their  bright  red  colour,  caused  by 
their  great  vascularity.  Towards  the  root  of  the  tongue 
we  find  the  third  kind  of  papillae,  the  circumvallate,  eight 
to  fifteen  in  number,  arranged  in  the  form  of  a  V,  with 
the  apex  directed  backwards.  Each  papilla,  surrounded 
by  a  deep  circular  furrow — hence  the  name — consists  of 
connective  tissue  clothed  with  epithelial  cells,  and  their 
height  varies  from  one-twenty-fifth  to  one-fifth  of  an  inch, 
and  their  breadth  from  one-twenty-fifth  to  one-eighth  of  an 


The  Sense  of  Taste 


7i 


inch.      It  is  in  connection  with  the  fungiform  and  circum- 
vallate  papillae  that  we  find  the  terminal  organs  of  taste. 

Minute  Structure  of  Gustatory  Organ. — In  many  of 
the  fungiform  and  in  all  the  circumvallate  papillae  are  the 
structures  called  taste  buds  or  taste  goblets.  They  also 
occur  to  a  small  extent  on  the  soft  palate,  and  even  on  the 
surface  of  the  epiglottis.  They 
are  most  conveniently  studied 
in  the  tongue  of  the  rabbit. 
Two  oval  patches  — ftaftillce 
foliatcB — may  be  seen  with  the 
naked  eye  near  the  root  of  the 
tongue  of  this  animal,  one  on 
each  side  and  placed  obliquely. 
Each  patch  consists  of  about 
twenty  laminae  or  folds  of  mucous 
membrane,  running  parallel, 
like  the  leaves  of  a  book,  and 
each  fold  is  composed  of  three 
ridges  of  the  derma.  Thus  a 
transverse  section  gives  the 
appearance  seen  in  Fig.  23. 

It  will  be  seen  that  the 
epithelium  is  thick  over  the  top 
and  thin  at  the  sides  of  the  fold, 
and  that,  in  section,  the  space 
between  two  folds  has  the 
appearance  of  a  deep  groove. 
About  the  middle  of  the  depth  of  this  groove  we  find  a 
row  of  minute  oval  bodies,  from  three  to  five  in  number 
— these  are  the  taste  buds,  or  taste  goblets.  They  exist  in 
immense  numbers.  In  the  papillae  foliatae  of  the  rabbit 
there  are  from  14,000  to  15,000,  while  the  tongues  of  the 
sheep  and  pig  have  yielded  9500,  and  that  of  the  ox  30,000 


Fig.  23. — Vertical  section  through  a 
portion  of  the  papilla  foliata  of  a 
rabbit  X  80  d.  Each  fold,  /,  has 
secondary  folds,  /' ;  g,  taste  goblets  ; 
n,  medullated  nerve  fibres ;  d,  a 
serous  gland ;  M,  muscular  fibres 
of  the  tongue.     (Stohr.) 


72  Physiology  of  the  Senses 

taste  buds.  As  many  as  1760  have  been  counted  on  one 
circumvallate  papilla  of  an  ox. 

The  taste  buds  are  oval  bodies,  one-three-hundredth  of 
an  inch  in  length  by  about  one-six-hundredth  of  an  inch  in 
breadth,  embedded  in  the  epithelial  layer.  The  base  rests 
on  the  derma,  while  the  other  and  somewhat  narrower 
end  is  directed  towards  the  sides  of  the  papilla  or  folds 
already  described,  and  shows  a  minute  funnel  -  shaped 
opening,  called  the  taste  pore.  Each  taste  bud  is  formed 
of  three  kinds  of  epithelial  cells  :  an  outer  set,  of  almost 
uniform  breadth  throughout,  and  shaped  somewhat  like  the 
staves  of  a  cask,  and  an  inner  of  two  varieties,  smaller  and 
pointed  at  each  end.  The  outer  cells — protecti?ig  cells — 
forming  the  outer  part,  are  evidently  structures  that  serve 
the  purpose  of  protecting  the  more  delicate  cells  in  the 
interior  of  the  little  flask.  There  appear  to  be  two  kinds 
of  inner  cells.  First,  we  find  cells  that  are  narrow  and 
slightly  thickened  in  the  middle,  where  there  is  a  nucleus, 
surrounded  by  only  a  very  small  amount  of  cell  substance. 
The  outer  half  of  the  cell  is  first  cylindrical,  then  conical, 
and  ends  in  a  fine  point,  while  the  inner  half  runs  deeply, 
sometimes  divides  into  two  roots,  and  is  lost  in  the  under- 
lying tissue.  Such  cells  have  been  termed  rod  cells,  and 
they  probably  support  the  true  sensory  cells  that  are  found 
in  the  middle  of  the  flask.  These — the  true  taste  cells — 
are  similar  in  appearance  to  the  rod  cells,  but  more  delicate  ; 
and  their  external  portions,  in  the  form  of  fine  threads, 
converge  so  as  to  form  a  tuft  at  the  taste  pore.  Both  the 
rod  cells  and  the  true  taste  cells  stain  with  chloride  of  gold, 
and  behave,  to  chemical  reagents,  like  sensory  cells. 

Terminations  of  Gustatory  Nerves. — As  to  the  way 
in  which  the  nerve  fibres  terminate  there  is  still  consider- 
able doubt.  The  fibres  of  the  glosso-  pharyngeal  nerve 
ramify  in  the  derma,  or  tissue  underlying  the  taste  buds, 


The  Sense  of  Taste 


73 


forming  plexuses  or  networks  from  which  minute  twigs 
pass  into  the  taste  buds.  Many  of  these  fibres  are  non- 
medullated.  Efforts  to  trace  them  into  connection  with 
the  true  taste  cells,  or  with  the  rod  cells,  have  failed,  but 
there  is  little  doubt  that  this  is  their  mode  of  termina- 
tion. Probably  some  fibres  may  not  enter  the  taste  buds 
at  all,  but  may  end  by  fine  processes  among  the  epithelium 
on  the  top  or  sides  of  the  papilla. 

The  proofs  that  the  taste  buds  are  the  end  organs  of 
taste  maybe  shortly  stated  as  follows:  (i)  The  sense  of 
taste  is  weakened  or  absent  in 
those  areas  of  mucous  mem- 
brane on  the  tongue  from  which 
they  are  absent  or  exist  only  in 
small  numbers  ;  (2)  the  sense 
is  most  acute  where  they  are 
found  in  large  numbers ;  (3) 
section  of  the  glosso-pharyngeal 


nerve,  which  is  distributed   to 


Fig.  24.  —  Taste  bud  seen  in  the 
papilla  foliata  of  a  rabbit  X  560  d. 
g,  Taste  bud,  showing  outer  sup- 
porting cells  ;  s,  fine  ends  of  taste 
cells  ;  p,  taste  pore.     (Stohr.) 


the  area  of  mucous  membrane 
where  taste   is  present,  is  fol- 
lowed  by  degeneration  of  the 
rod  and  taste  cells,  and  ultimately  by  the  entire  disappear- 
ance of  the  taste  bud. 

Physical  Causes  of  Taste. — All  substances  that  give 
rise  to  taste  are  soluble  in  the  fluids  of  the  mouth.  In- 
soluble substances  are  tasteless.  Thus,  if  we  touch  the 
surface  of  a  crystal  of  quartz  with  the  tongue,  we  have  a 
sensation  of  smooth  contact,  or  touch,  and  a  sensation  of 
cold,  because  the  crystal  conducts  heat  out  of  the  tongue, 
but  there  is  no  sense  of  taste.  Contrast  this  with  the 
sensations  of  saline  taste,  contact,  and  coolness  experienced 
when  we  bring  the  tongue  into  contact  with  the  surface  of 
a  crystal  of  rock  salt.     As  solution  is  a  necessary  condi- 


74  Physiology  of  the  Senses 

tion  of  taste  we  find  near  the  taste  organs  numerous  small 
serous  or  albuminous  glands  (see  Fig.  23),  the  secretions 
of  which  assist  in  dissolving  sapid  substances.  No  con- 
nection has  yet  been  traced  between  the  chemical  composi- 
tion of  sapid  substances  and  the  different  kinds  of  tastes  to 
which  they  give  rise.  Substances  of  very  different  chemical 
composition  may  give  rise  to  similar  tastes.  For  example, 
sugar,  acetate  of  lead,  and  chloroform  have  all  a  sweetish 
taste,  although  their  chemical  composition  is  as  diverse  as 
can  well  be  imagined.  Acids  are  usually  sour ;  alkalies 
have  a  peculiar  soapy  taste  ;  salts  vary  much,  from  the 
sweetness  of  sugar  of  lead  to  the  bitterness  of  sulphate  of 
magnesia ;  the  soluble  alkaloids,  such  as  quinine,  strych- 
nine, etc.,  are  usually  bitter ;  and  the  higher  alcohols  are 
more  or  less  sweet. 

Physiological  Conditions  of  Taste. — The  tongue,  as 
already  pointed  out,  is  the  seat  of  sensations  that  are  quite 
unlike  each  other.  Thus,  there  are  tactile  sensations,  as 
when  we  touch  the  organ  with  a  pin,  sensations  of  pressure, 
sensations  of  heat  and  of  cold,  burning  or  acrid  sensations, 
peculiar  sensations  excited  by  the  application  to  the  tongue 
of  an  interrupted  electrical  current,  and,  lastly,  sensations 
of  true  tastes.  We  must  also  distinguish  from  these,  sensa- 
tions that  are  called  flavours,  experienced  when  we  bring 
the  tongue  into  contact  with  an  onion  or  a  savoury  bit  of 
cooked  meat  or  fish.  These  are  in  reality  sensations 
compounded  of  smells  and  tastes,  and  the  sensation  of 
tasting  an  onion  is  thus  quite  changed  when  we  hold  the 
nose  and  avoid  breathing.  True  tastes  may  be  classified 
as  sweet,  bitter,  salt,  sour,  alkaline,  and,  perhaps,  metallic. 
All  of  these  are  specifically  distinct  sensations,  and  they 
are  no  doubt  due  to  some  kind  of  action,  probably  chemical, 
which  they  excite  in  the  taste  cells.  If  we  assume  that 
the  taste  cells  are  connected  with  the  ends  of  the  nerves, 


The  Sense  of  Taste  75 

then  we  can  imagine  that  the  chemical  changes  thus  excited 
in  the  taste  cells  set  up  nerve  currents  which,  propagated 
to  specific  centres  of  taste  in  the  brain,  give  rise  there  to 
molecular  changes  that  in  turn  are  related  to  consciousness. 

While,  however,  chemical  action  probably  lies  at  the 
root  of  the  mechanism  of  taste,  it  is  remarkable  that  true 
tastes  may  be  excited  by  causes  that  are  not  strictly 
chemical.  Thus  a  smart  tap  on  the  tongue  may  excite  a 
taste;  and  Siilzer  demonstrated,  so  long  ago  as  1752,  that 
a  constant  current  causes  (more  especially  at  the  moments 
of  opening  and  of  closing  the  current)  a  sensation  of 
acidity  at  the  anode  (positive  pole)  and  of  alkalinity  at  the 
kathode  (negative  pole).  No  doubt  it  is  possible  that  the 
mechanical  irritation,  in  the  one  case,  and  the  electrical 
current,  by  electrolysis,  in  the  other,  may  set  free  chemical 
stimuli ;  but  of  this  there  is  no  proof.  On  the  other  hand, 
it  has  been  found  that  sensations  of  taste  may  be  excited 
by  rapid  induction  currents — currents  too  rapid  to  produce 
electrolytic  action. 

The  extent  of  surface  acted  on  increases  the  massiveness 
of  the  sensation  of  taste,  while  the  intensity  is  influenced 
by  the  degree  of  concentration  of  the  solution  of  the  sapid 
substance.  Suppose  we  gradually  dilute  solutions  with  water, 
tasting  from  time  to  time,  until  no  taste  is  experienced, 
some  common  substances  may  be  classed  in  the  following 
order :  syrup,  sugar,  common  salt,  aloes,  quinine,  sulphuric 
acid.  That  is  to  say,  the  sweetness  of  syrup  disappears 
first,  and  the  sourness  of  sulphuric  acid  last.  Again,  it  has 
been  found  that  the  taste  of  quinine  continues  until  diluted 
with  twenty  times  more  water  than  common  salt.  It  is 
evident,  then,  that  smaller  quantities  of  some  substances,  as 
compared  with  others,  excite  taste,  or,  in  other  words,  the 
taste  cells  are  more  susceptible  to  the  chemical  action  of 
some  substances  than  of  others.     Attempts  have  been  made 


76  Physiology  of  the  Senses 

to  measure  the  time  required  to  excite  tastes.  Thus,  from 
the  moment  of  contact  with  the  tongue,  saline  matters  are 
tasted  more  rapidly  (.17  second)  than  sweet,  acid,  and 
bitter  (.258  second) — the  difference  being  probably  due  to 
the  activity  of  diffusion  of  the  substance.  After  a  taste  has 
been  developed,  it  appears  to  last  for  relatively  a  long 
time,  but  it  is  not  easy  to  say  whether  this  is  due  to  a  per- 
sistent change  in  the  taste  cells,  after  removal  of  the  exciting 
cause,  or  to  the  continued  action  of  the  exciting  substance. 
It  is  well  known  that  a  temperature  of  about  400  F.  is 
most  favourable  to  the  development  of  tastes,  fluids  much 
above  or  below  this  temperature  either  masking  or  tem- 
porarily paralysing  the  taste  cells.  Thus,  if  the  mouth  be 
rinsed  with  either  very  hot  or  very  cold  water,  a  solution  of 
sulphate  of  quinine,  distinctly  bitter  at  a  temperature  of 
400  F.,  will  scarcely  be  perceived. 

As  one  would  expect  from  the  anatomical  distribution 
of  the  taste  buds,  the  surface  of  the  tongue  is  not  uniformly 
sensitive  as  regards  taste.  The  sense  is  most  acute  in  or 
near  the  circumvallate  papillae.  The  middle  of  the  tongue 
is  scarcely  sensitive  to  taste,  while  the  edges  and  the  tip 
are,  as  a  rule,  highly  sensitive,  although  it  is  said  that  the 
sensitiveness  of  the  edges  varies  much  in  different  indivi- 
duals. Taste  is  feebly  developed  on  the  soft  palate  and  on 
the  pillars  of  the  fauces,  so  that  after  complete  extirpation 
of  the  tongue,  including  the  part  bearing  the  circumvallate 
papillae,  feeble  sensations  may  still  remain. 

Differentiation  of  Tastes. — Recent  observations  by  Shore1 
have  thrown  light  on  the  question  whether  there  may  be 
in  the  tongue  different  end-organs  appropriated  to  special 
tastes.  If  all  the  taste  buds  are  the  same,  it  is  difficult  to 
explain  why,  in  the  majority  of  persons,  the  back  part  of 
the  tongue  is  most  sensitive  to  bitters  and  the  tip  to  sweets, 
1  Shore,  //.  of  Physiology,  1891. 


The  Sense  of  Taste  77 

why  saline  matters  are  perceived  most  distinctly  at  the  tip 
and  acid  substances  at  the  sides,  and  why  there  should  be 
individual  variations,  as  undoubtedly  is  the  case.  Assuming 
that  there  are  different  kinds  of  taste  cells,  it  might  be  possible 
to  paralyse  some  without  affecting  others,  and  thus  different 
sensations  of  tastes  might  be  discriminated.  This  has  been 
done  by  the  use  of  the  leaves  of  a  common  Indian  plant, 
Gymnema  sylvestre.  If  some  of  these  be  chewed,  it  has 
been  found  that  bitters  and  sweets  are  paralysed,  while 
acids  and  salines  are  unaffected.  Again,  certain  strengths 
of  decoctions  of  the  leaves  appear  to  paralyse  sweets 
sooner  than  bitters.  These  interesting  observations  indicate 
the  existence  of  different  taste  cells  for  sweets,  bitters, 
acids,  and  salines  ;  and  it  is  clear  that  the  region  of  the 
tongue  most  richly  supplied  with  taste  cells  sensitive  to 
sweets  will  respond  best  to  sweet  substances,  while  another 
region,  supplied  by  taste  cells  sensitive  to  bitters,  will  respond 
best  to  bitter  substances.  In  like  manner  the  argument 
may  be  applied  to  other  tastes.  Suppose,  again,  a  set  of 
taste  cells  sensitive  to  bitter  substances  :  it  is  conceivable 
that  in  whatever  way  these  were  irritated,  a  bitter  taste 
would  result.  If  so,  a  substance  which  applied  to  one  part 
of  the  tongue  would  cause  a  sweet  sensation,  might  cause  a 
bitter  if  applied  to  a  part  of  the  tongue  richly  supplied  with 
taste  cells  sensitive  to  bitters.  This  may  explain  why  sul- 
phate of  magnesia  excites  at  the  root  of  the  tongue  a  bitter 
taste,  while  applied  to  the  tip  it  causes  a  sweet  or  acid 
taste.  Saccharine,  in  like  manner,  is  sweet  to  the  tip  and 
bitter  to  the  back  of  the  tongue.  Again,  it  has  been  found 
that  if  the  "sweet"  and  "bitter"  taste  cells  are  paralysed 
by  gymnema,  electrical  irritation  of  the  tip  does  not  give 
rise  to  an  acid  taste  mixed  with  sweet,  but  to  sensations 
somewhat  different,  and  described  as  "metallic,"  or  "salt," 
or  "  acid." 


78  Physiology  of  the  Senses 

General  Seiisibility  of  the  To?igue. — As  already  said,  the 
tongue  is  endowed  with  acute  general  sensibility.  It  is 
evident,  then,  that  a  sensation  caused  by  dropping  a  little 
vinegar  on  the  tongue  is  due  partly  to  stimulation  of  the 
tactile  organs,  and  partly  to  stimulation  of  the  true  taste  cells. 
Cocaine,  the  active  alkaloid  of  the  coca  plant,  paralyses 
tactile  sensibility ;  and  it  is  said  that  if  the  surface  of  the 
tongue  be  painted  with  a  solution  of  this  substance,  that 
acid  tastes  become  more  clear  and  marked.  The  drug,  how- 
ever, ultimately  affects  all  the  end  organs,  so  that  lingual 
sensations  disappear  in  the  following  order :  general  sensi- 
bility and  pain,  bitters,  sweets,  salines,  acids,  and  tactile 
sensibility. 

Stibjective  Tastes. — Disease  of  the  tongue  causing  un- 
natural dryness  may  affect  taste.  Substances  circulating 
in  the  blood  sometimes  give  rise  to  subjective  sensations  of 
taste.  Thus  santonine,  morphia,  and  biliary  products,  as  in 
jaundice,  cause  a  bitter  sensation,  while  in  diabetes  there 
is  often  a  persistent  sweetish  taste.  The  insane  occasionally 
suffer  from  distressing  subjective  tastes.  In  such  cases  the 
sensation  is  caused  by  irritation  of  the  gustatory  nerve,  or 
by  changes  in  the  taste  centres  of  the  brain.  There  is,  how- 
ever, no  evidence  showing  that  direct  irritation  of  gustatory 
nerves  is  followed  by  sensations  of  taste. 

Nerves  of  the  Tongue. — The  distribution  of  nerves  to  the 
tongue  is  remarkably  complicated,  and  the  whole  subject 
presents  numerous  difficulties.  The  motor  nerve,  that  is, 
the  nerve  that  excites  and  governs  the  movement  of  the 
tongue,  is  the  ninth  cranial  nerve,  known  to  anatomists  as 
the  hypo-glossal.  The  sensory  nerves  are  usually  described 
as  two  in  number,  the  anterior  two-thirds  of  the  tongue 
being  supplied  by  the  gustatory  or  lingual  branch  of  the 
fifth  cranial  nerve,  and  the  posterior  third — the  situation  of 
the  circumvallate  papillae — by  the  glosso-pharyngeal  nerve. 


The  Sense  of  Taste  79 

The  lingual  branch  of  the  fifth  nerve  contains  both  ordinary 
sensory  and  gustatory  filaments,  and  the  glosso-pharyngeal 
supplies  the  circumvallate  papillae  and  taste  buds.  Another 
nerve,  however,  has  to  be  considered,  namely,  the  chorda 
tympani,  a  branch  given  off  by  the  facial  nerve  during  the 
passage  of  the  latter  through  a  canal  in  the  petrous  portion 
of  the  temporal  bone  known  as  the  aqueduct  of  Fallopius. 
Loss  of  taste  on  one  side  of  the  tongue  has  been  observed 
in  cases  of  disease  of  the  ear  involving  the  chorda  nerve. 
This,  however,  is  not  conclusive  evidence  that  the  chorda 
contains  gustatory  filaments,  as  the  loss  of  taste  following 
its  injury  may  be  due  to  the  removal  of  its  influence  over 
the  nutrition  of  the  mucous  membrane  of  the  organ.  On 
the  other  hand,  there  are  good  grounds  for  the  view  that 
the  gustatory  filaments,  both  of  the  lingual  branch  of  the 
fifth,  and  of  the  glosso-pharyngeal  itself,  come  primarily 
from  the  roots  of  the  fifth  nerve.  Disease  of  this  nerve 
within  the  cranial  cavity  causes  loss  of  taste  in  one  lateral 
half  of  the  tongue,  both  tip  and  back,  but  no  case  has  been 
recorded  of  disease  of  the  glosso-pharyngeal  being  followed 
by  this  result. 


THE  SENSE  OF  SMELL 

The  seat  of  the  structures  concerned  in  the  sense  of  smell 
is  in  the  nasal  cavities,  situated  between  the  base  of  the 


Fig.  25. — Transverse  vertical  section  across  the  nasal  cavities,  opposite  to  the 
middle  of  the  hard  palate  ;  the  anterior  part  of  the  section  seen  from  behind. 
1,  Part  of  inner  surface  of  cranium ;  2,  projection  between  the  two  cribri- 
form plates  of  the  ethmoid  bone  ;  3,  median  septum  or  partition  in  the 
ethmoid  bone  ;  4,  4,  cells  in  the  lateral  masses  of  the  ethmoid  bone  ;  5,  5, 
the  middle  turbinated  portion  of  the  ethmoid  bone  ;  6,  6,  the  two  turbin- 
ated bones  ;  7,  the  vomer,  or  bony  septum  or  partition,  of  the  nose  ;  8, 
section  of  the  malar  or  cheek-bone  ;  9,  a  large  sinus  or  space  in  the  superior 
maxillary  bone — sometimes  called  the  maxillary  sinus,  or  antrum  of  High- 
more  ;  it  communicates  with  the  nasal  cavity,  at  10,  and  there  is  a  corre- 
sponding space  on  the  other  side.     (Arnold.) 

cranium  and  the  roof  of  the  mouth,  at  the  upper  and  fore 
part  of  the  face.     The  floor,  sides,  and  roof  of  these  cavities 


The  Sense  of  Smell  81 

are  formed  by  certain  of  the  bones  of  the  cranium  and  face 
(see  Fig.  25). 

Physiological  Anatomy  of  the  Nose. — The  ethmoid 
bone,  which  also  forms  part  of  the  floor  of  the  cranial  cavity, 
is  concerned  in  the  formation  of  the  olfactory  region.  Thus 
its  cribriform  plates  form  the  roof;  its  sides,  which  contain 
numerous  cavities  or  cells  formed  of  bone,  constitute  the 
convoluted  sides  of  the  upper  part  of  the  cavity ;  and  a 
median  plate  of  bone,  forming  a  septum  or  partition,  assists 
in  dividing  the  one  nasal  cavity  from  the  other.  The 
anterior  part  of  the  nasal  cavities  is  completed  at  the  sides 
and  in  the  middle  by  plates  of  cartilage  or  gristle,  called  the 
nasal  cartilages.  These  cartilages  are  firmly  attached  to  the 
margin  of  the  nasal  aperture  seen  in  a  skull,  and  they  give 
form  and  firmness  to  the  visible  part  of  the  nose. 

The  nostrils  open  anteriorly  by  apertures  called  the 
anterior  ?tares,  and  they  are  lined  by  an  infolding  of  skin, 
bearing  short  stiff  hairs,  vibrissa,  which,  to  some  extent, 
prevent  the  entrance  of  foreign  bodies.  Posteriorly,  the 
nostrils  open  into  the  pharynx  by  two  apertures,  the  posterior 
nares  (see  Fig.  26).  The  middle  wall  of  each  nostril  is 
formed  by  the  septum  or  partition  between  the  two,  and 
presents  a  smooth  surface.  The  outer  wall,  on  the  other 
hand,  is  more  or  less  convoluted  from  the  presence  of  three 
delicate  scroll-like  bones,  namely,  the  upper  and  middle  tur- 
binated parts  of  the  ethmoid,  and  the  lower  turbinated  bones 
(see  Fig.  25,  5,  6).  There  are  thus  three  spaces,  or  recesses, 
called  the  superior,  middle,  and  inferior  meatus,  and  these 
meatuses  communicate  with  cavities,  called  sinuses,  in  the 
ethmoid,  sphenoid,  frontal,  and  upper  jaw-bones.  These 
spaces,  along  with  the  cavity  of  the  nose  itself,  being  full  of 
air,  act  as  resonators,  and  affect  the  quality  of  the  voice. 

The  cavity  of  the  nose  is  lined  by  a  membrane,  called 
the  nasal  mucous  membrane,    or   Schneiderian   membra?ie, 

G 


82 


Physiology  of  the  Senses 


which  secretes  a  peculiar  kind  of  mucus  known  as  pituita. 
The  lining  membrane  is  continuous  with  that  of  the  sinuses 
already  mentioned,  and  with  the  lining  of  the  pharynx  and 
Eustachian  tube,  while  it  is  prolonged  on  each  side,  through  a 
small  canal,  into  the  lachrymal  sac,  thus  also  merging  into 
the  conjunctiva,  the  mucous  membrane  of  the  eye-lids. 


Fig.  26. — Outer  side  of  left  naris.  1,  Sinus  or  hollow  in  the  frontal  bone ;  2, 
free  border  of  the  nasal  bone  ;  3,  lamina  cribrosa  or  perforated  plate  of 
ethmoid  bone,  through  which  pass  the  twigs  of  the  olfactory  nerve ;  4,  an- 
trum or  hollow  of  the  sphenoid  bone  ;  5,  hairs  in  the  vestibule  of  the  nose  ; 
6,  6',  vestibule  of  the  nose  separated  by  a  prominence,  7,  from  8,  the  entrance 
to  the  middle  meatus  or  passage  of  the  nose  ;  9,  agger  or  mound  of  the  nose, 
the  rudiment  of  a  muscle;  10,  concha  or  shell  of  Santorini ;  11,  entrance 
to  4 ;  12,  superior  spongy  bone ;  13,  upper  meatus ;  14,  middle  spongy 
bone ;  its  inferior  free  border  from  b  \.o  c;  15,  inferior  spongy  bone  ;  16, 
naso-pharyngeal  fold  ;  17,  naso-pharyngeal  duct ;  18,  pharyngeal  opening 
of  the  Eustachian  tube  ;  19,  fold  between  18  and  pharynx  ;  20,  depression 
of  Rosenmiiller  ;  21,  the  incisor  canal.     (Schwalbe.) 

The  nerves  supplying  the  nasal  mucous  membrane  come 
from  three  sources.  First,  it  is  supplied  by  the  nasal  and 
anterior  dental  branches  of  the  fifth  pair  of  cranial  nerves  ; 
second,  branches  are  distributed  to  it  from  the  vidian,  naso- 
palatine, descending  palatine,  and  spheno-palatine  nerves, 
in  which  run  fibres  of  the  sympathetic  ;  and,  third,  we  find 


The  Sense  of  Smell  83 

in  the  upper  part  of  the  nasal  cavities  branches  of  the  first 
pair  of  cranial  nerves,  the  olfactory  nerves.  The  first  two 
groups  of  nerves  endow  the  nose  with  general  sensibility, 
and  supply  its  blood-vessels  and  glands.  The  olfactory 
nerves  are  the  true  nerves  of  smell,  and  their  branches  end 
in  the  special  terminal  organs  devoted  to  that  sense. 

The  olfactory  lobes  (see  Fig.  8,  p.  21)  lie  within  the 
cranium  on  the  cribriform,  or  sieve -like,  plates  of  the 
ethmoid  bone,  and  about  twenty  small  branches,  the 
olfactory  7ierves,  issue  from  their  under  surface,  pass 
through  minute  canals  in  the  ethmoid  bone,  and  thus  gain 
the  upper  part,  or  roof,  of  the  nasal  cavities.  There  they 
divide  into  three  groups,  one  supplying  the  roof,  a  second 
the  membrane  covering  the  cellular  part  of  the  ethmoid 
bone,  while  the  third  pass  as  low  as  the  middle  turbinated 
bone.  Some  fibres  also  reach  ancl  are  distributed  to  the 
upper  third  of  the  nasal  septum. 

The  nasal  mucous  membrane  is  richly  supplied  with 
blood,  a  dense  capillary  network  lying  below  the  epithelial 
layer.  The  veins  converge  to  the  posterior  part  of  the 
lower  meatus,  where  they  form  a  thick  dense  plexus.  The 
existence  of  so  many  vessels  accounts  for  the  nasal  haemor- 
rhage often  observed,  and  as  the  bleeding  not  unfrequently 
proceeds  from  the  venous  plexus  situated  far  back  in  the 
cavities,  it  is  sometimes  staunched  with  difficulty. 

Minute  Structure  of  the  Olfactory  Organ. — As  already 
mentioned,  the  membrane  lining  the  movable  (anterior) 
part  of  the  nose  is  developed  from  an  infolding  of  the  skin, 
and  in  structure  it  resembles  skin,  showing  a  layer  of 
stratified  squamous  epithelium  covering  papillae.  In  it  we 
find  numerous  sebaceous  -  glands  and  hair  follicles,  from 
which  vibrissa  spring.  This  part  of  the  nose,  the  vesti- 
bular portion,  is  at  the  entrance  of  the  respiratory  passage. 
The  respiratory  fiortio?i  forms  the  lower  part  of  the  nasal 


84 


Physiology  of  the  Senses 


Epithelium. 


passage.  It  is  lined  by  a  stratified  cylindrical  epithelium, 
the  cells  of  which  bear  cilia,  short  vibratile  processes,  by 
the  movements  of  which  currents  are  established  in  the 
fluid  bathing  the  surface.  In  this  portion,  the  membrane 
of  which  is  about  one-sixth  of  an  inch  in  thickness,  are 

numerous  minute  race- 
mose glands  secreting  a 
fluid,  thus  keeping  the 
surface  moist,  and  it  is 
noticeable  that  in  the 
sinuses  already  mentioned 
the  membrane  is  much 
thinner,  and  only  very 
few  glands  exist. 

The  upper,  or  olfactory 
ftortiofi)  is  the  part  spe- 
cially connected  with  the 
sense  of  smell.  It  is 
narrow  from  side  to  side, 
and  clothed  with  a  thick 
mucous  membrane,  often 
of     a     yellowish  -  brown 

Fig.  27.— Vertical  section  through  the  olfac-  Colour,  that  Contrasts  with 

tory  region  of  a  rabbit,  magnified  560  dia-  the     reddish    hue    0f  that 
meters,     s,  Border ;  zo,  zone  of  the  oval 

nuclei;  zr,  zone  of  the  round  nuclei;  b,  lining    the   vestibular   and 

basal   cells;    dr,    portions    of   Bowman's  respiratory      regions.  A 
glands.    The  lower  part  of  the  duct  is  seen           r                       . 

on  the  right,    n,  branch  of  olfactory  nerve,  vertical      Section     of  this 

(Stohr.)  membrane  is  seen  in  Fig. 

27.  It  is  formed  of  an  epithelial  layer,  olfactory  epithelium, 
resting  on  a  basement  membrane.  Two  forms  of  cells  are 
found.  The  one  (Fig.  28,  st)  has  the  upper  half  cylindrical, 
and  the  free  border  sometimes  shows  minute  stiff  cilia, 
while  the  lower  half  is  narrowed,  shows  indentations,  and 
finally   ends   in   long,  sometimes   double,  processes,  which 


Mucous 

membrane 


The  Sense  of  Smell 


85 


apparently  join  with  those  of  adjoining  cells.  These  knife- 
handle-like  cells,  called  supporting  cells,  show  each  an  oval 
nucleus,  and  the  rows  of  such  nuclei,  seen  in  a  section,  as 
in  Fig.  2  7,  form  a  zone,  known  as  the  zone  of  oval  ?iuclei. 
The  second  cells  have  a  round  nucleus  surrounded  by  only 
a  small  quantity  of  protoplasm,  and  from  this  there  passes 
to  the  surface  a  narrow  round  filament, 
bearing  a  single  cilium  on  its  free  end,  while 
another  slender  filament  passes  in  the  oppo- 
site direction,  and  terminates  in  filaments 
of  the  olfactory  nerve.  These  are  the  olfac- 
tory cells.  The  juxtaposition  of  the  round 
nuclei  forms  a  zone,  called  the  zone  of  roimd 
nuclei.  At  the  boundary  of  the  epithelial 
layer  with  the  connective  tissue,  peculiar, 
somewhat  flattened  or  irregularly  cubical 
cells  are  found,  termed  basal  cells  (Fig.  27,  b).  Fig.  28.— isolated 
The  layer  on  which  the  epithelium  rests  is 
a  loose  felt  work  of  connective  tissue,  con- 
taining elastic  fibres,  and  the  latter  may  be 
so  close  together  as  to  form  an  elastic  layer. 
Numerous  simple  or  branched  glands  exist 
in  the  olfactory  region,  named  after  their 
discoverer  the  glands  of  Bowman.  They 
secrete  mucus,  but  their  special  function  is 
unknown. 

As  to  the  mode  of  termination  of  the 
olfactory  nerves  there  is  still  considerable 
difference  of  opinion,  some  holding  that  they  end  only  in 
the  true  olfactory  cells  (Fig.  28,  r),  while  others  maintain 
that  they  also  end  in  the  basal  cells  (Fig.  27,  £),  and  even 
in  the  supporting  cells  (Fig.  27,  s,  and  Fig.  28,  st).  The 
evidence  is  clear  that  they  end  in  the  olfactory  cells,  but 
doubtful  as  regards  the  others,  and,  from  the  analogy  of 


cells  from  the  ol- 
factory region  of  a 
rabbit,  magnified 
560  diameters,  st, 
Supporting  cells  ; 
s,  short,  stiff  cilia, 
or,  according  to 
some,  cones  of 
mucus  resemb- 
ling cilia  ;  r,  r,  ol- 
factory cells.  The 
nerve  process  has 
been  torn  off  the 
lower  cell  marked 
r.     (Stohr.) 


86  Physiology  of  the  Senses 

other  end  organs,  it  is  probable  that  the  basal  and  support- 
ing cells  have  only  indirectly  to  do  with  the  action  of  odori- 
ferous substances  on  the  nerve-endings. 

Physical  Causes  of  Smell. — Substances  that  excite  the 
sense  of  smell  must  exist  in  the  atmosphere  in  a  state  of 
fine  subdivision,  and  even  vapours  and  gases  may  be 
supposed  to  consist  of  minute  molecules  of  matter.  If  air 
conveying  an  odour  be  passed  through  a  long  glass  tube 
packed  firmly  with  cotton  wool,  it  will  still  be  odorous, 
although  this  proceeding  will  remove  all  particles  larger 
than  the  one  -  hundred  -  thousandth  of  an  inch.  Again, 
a  grain  of  musk  will  for  years  communicate  its  odour  to 
the  air  of  a  room,  and  at  the  end  of  the  time  it  will 
not  have  appreciably  diminished  in  weight.  Odoriferous 
particles  will  mix  with  the  air  either  in  accordance  with 
the  laws  of  diffusion  of  gases  or  by  virtue  of  their  volatility, 
that  is,  the  rapidity  with  which  they  evaporate.  4n  the 
case  of  odorous  gases,  no  doubt  mixture  takes  place  by 
diffusion,  but  an  odorous  essential  oil  will  give  off  particles 
by  a  kind  of  evaporation.  The  volatility  of  a  substance 
may  be  expressed  by  the  weight  that  evaporates  from  a 
unit  of  surface  in  a  unit  of  time.  By  means  of  a  specially- 
contrived  instrument  Ch.  Henry  has  measured  the  volatility 
of  various  odorous  substances,  and,  as  might  be  expected,  it 
is  very  great.  Thus,  taking  unity  as  the  one-thousandth  of 
a  milligramme  i  evaporating  from  one  square  millimetre  in 
one  second,  the  following  values  were  obtained  :  ether,  .7  ; 
ylang-ylang,  .0176;  rosemary,  .0446;  caraway,  .0315; 
mint,  .0354;  winter-green,  .0165  ;  bergamot,  .0331  ;  and 
lavender,  .0292.  These  minute  quantities  are  readily 
appreciated  by  the  sense  of  smell,  if  the  nose  is  held  near 
the  evaporating  surface. 

1  The  one-thousandth  of  a  milligramme  =  one  twenty-five-millionth 
of  a  grain. 


The  Sense  of  Smell  87 

Chemical  nature  of  odorous  substances. — Attempts  have 
been  made,  notably  by  Ramsay  and  Haycraft,  to  dis- 
cover a  relation  that  might  exist  between  odours  and  the 
chemical  composition  of  substances  emitting  them.  Certain 
gases  excite  smell,  while  others  are  odourless.  Thus 
the  following  having  no  smell  :  hydrogen,  oxygen,  nitrogen, 
water  gas,  marsh  gas,  olefiant  gas,  carbon  monoxide,  hydro- 
chloric acid,  formic  acid,  nitrous  oxide,  and  ammonia.  It 
is  necessary,  of  course,  to  distinguish  between  the  irritant 
action  of  such  gases  as  ammonia  and  hydrochloric  acid,  and 
the  true  odour.  On  the  other  hand,  the  following  gases 
have  an  odour:  chlorine,  bromine,  and  iodine,  the  compounds 
of  chlorine  and  bromine  with  oxygen  and  water,  peroxide  of 
nitrogen,  the  vapours  of  sulphur  and  phosphorus,  arsenic, 
antimony,  sulphurous  acid,  carbonic  acid,  some  compounds 
of  selenium  and  tellurium,  the  compounds  of  chlorine, 
bromine,  and  iodine,  with  the  above-named  metals,  and 
many  of  the  volatile  compounds  of  carbon.  Substances  of 
low  molecular  weight  either  simply  irritate  the  nose,  or 
have  no  odour.  Ramsay  states  that  in  the  carbon  com- 
pounds increase  of  specific  gravity  as  a  gas  is  related  (up 
to  a  certain  point)  to  smell.  Thus,  if  we  take  the  methane 
or  marsh  gas  series  (the  paraffins),  the  first  two  have  no 
smell,  ethane  (fifteen  times  as  heavy  as  hydrogen)  has  a 
faint  odour,  and  it  is  not  till  we  reach  butane  (thirty  times 
heavier  than  hydrogen)  that  a  distinct  odour  is  noticeable. 
Again,  methyl  alcohol  has  no  smell  ;  ethyl,  or  ordinary 
alcohol,  has  a  true  alcoholic  smell,  "  and  the  odour  rapidly 
becomes  more  marked  as  we  rise  in  the  series,  till  the 
limit  of  volatility  is  reached,  and  we  arrive  at  solids  with 
such  a  low  vapour  tension  that  they  give  off  no  appreciable 
amount  of  vapour  at  the  ordinary  temperature."  1  Again, 
acids  increase  in  odour  with  an  increase  in  density  as  a 
1  Ramsay,  Nature,  vol.  xxvi.  p.  187. 


88  Physiology  of  the  Senses 

gas.  Formic  acid,  for  example,  has  no  smell ;  acetic  acid 
has  its  well-known  odour  of  vinegar  ;  and  propionic,  butyric, 
and  valerianic  acids  increase  in  odour  as  we  ascend  the 
series.  Groups  of  chemical  substances  have  sometimes 
characteristic  smells.  Thus  many  compounds  of  chlorine, 
sulphur,  selenium,  tellurium,  the  paraffins,  alcohols,  nitrites, 
amines,  the  pyridenes,  and  the  benzene  group  have  each  a 
characteristic  odour.  Again,  substances  not  related,  but 
similar  in  chemical  structure,  may  have  somewhat  similar 
odours.  Thus  the  compounds  of  hydrogen  with  sulphur, 
selenium,  and  tellurium,  and  the  compounds  of  these  with 
methyl  or  ethyl,  have  all  a  disagreeable  odour,  something 
like  that  of  garlic.  The  odours  of  chloroform  and  iodoform 
are  not  unlike. 

On  the  other  hand,  many  substances  have  odours  that 
are  very  similar,  and  yet  there  is  no  resemblance  in  chemical 
constitution.  Why,  for  example,  should  arsenical  .com- 
pounds have  the  odour  of  garlic  ?  Why  have  nitro-benzene, 
benzoic  aldehyde,  and  prussic  acid  almost  the  same  odour  ? 
Mix  sulphuric  aci'd  with  water,  and  an  odour  like  that  of 
musk  may  be  given  out.  It  is  said  that  emeralds,  rubies, 
and  pearls  if  triturated  for  a  long  time  give  out  an  odour  like 
that  of  violets.  Again,  the  disease  called  favus,  ringworm 
of  the  scalp,  the  body  of  a  patient  sick  of  typhus,  and  mice 
have  similar  odours.  It  is  well  known  that  perfumes 
from  very  different  sources  may  be  classed  under  certain 
types.  Thus,  the  rose  type  includes  geranium,  eglantine, 
and  violet-ebony  ;  the  jasmine  type,  lily  of  the  valley  and 
ylang-ylang ;  the  orange  type,  acacia,  seringa,  and  orange- 
flower  ;  the  vanilla  type,  balsam  of  Peru,  benzoin,  storax, 
tonka  bean,  and  heliotrope  ;  the  lavender  type,  thyme  and 
marjoram  ;  the  mint  type,  peppermint,  balsam,  and  sage  ; 
the  musk  type,  musk  and  amber  seed  ;  and  the  fruity  type, 
pear,  apple,  pine-apple,  and  quince. 


The  Sense  of  Smell  89 

Flowers  and  odours. — Attempts  have  also  been  made  to 
discover  a  relation  between  the  colours  of  flowers  and  the 
intensity  of  their  perfumes.  White  flowers  manifest  the 
greatest  variety  of  odours,  and  then  follow  reds,  yellows, 
greens,  and  blues.  The  ratio  of  the  number  of  odorous 
species  to  the  number  of  species  in  each  colour,  is  as 
follows:  whites,  1  to  6.37;  reds,  1  to  10.8;  yellows,  1  to 
12.6;  greens,  1  to  12.7;  and  blues  1  to  19.  It  is  also 
noticeable  that  flowers  which  by  their  colour  emit  most 
heat  will  volatilise  the  greatest  amount  of  perfume,  and  that 
the  more  refrangible  the  rays  reflected  from  the  flower  the 
smaller  is  the  amount  of  perfume.  Coloured  substances 
have  also  different  powers  of  absorbing  odours.  Whites, 
yellows,  reds,  greens,  and  blues  absorb  odours  on  a  decreas- 
ing scale.  The  more  intense  the  colour  the  more  likely  is 
it  to  emit  a  strong  odour,  because  no  doubt  the  light  acts 
on  the  essential  oil  on  which  the  odour  depends.  Heat 
more  than  light  favours  the  volatilisation  of  perfumes. 
Hence  the  odours  of  a  flower-bed  in  a  garden  are  often  most 
apparent,  not  in  bright  sunshine,  but  in  the  shade.  Some 
essential  oils  require  a  higher  temperature  than  others  to 
bring  out  their  characteristic  perfumes.  An  air  of  moderately 
high  temperature  and  the  presence  of  moisture  favour  the 
diffusion  of  the  odours  of  most  flowers. 

Odours  and  heat  absorption. — Tyndall  showed  that  many 
odorous  vapours  have  a  considerable  power  of  absorbing 
heat.  Taking  the  absorptive  capacity  of  air  as  unity,  the 
absorption  per  cent,  for  certain  odorous  matters  was  as 
follows  :  patchouli,  30  ;  sandal-wood,  32  ;  geranium,  ^2>  > 
oil  of  cloves,  33.5  ;  otto  of  roses,  36.5  ;  bergamot,  44  ; 
neroli,  47  ;  lavender,  60  ;  lemon,  65  ;  portugal,  67  ;  thyme, 
68  ;  rosemary,  74  ;  oil  of  laurel,  80;  and  cassia,  109.  In 
comparison  with  the  air  introduced  in  the  experiments  the 
weight  of  the  odours  was  extremely  small.      "  Still  we  find 


90  Physiology  of  the  Senses 

that  the  least  energetic  in  the  list  produces  thirty  times  the 
effect  of  air,  while  the  most  energetic  produces  one  hundred 
and  nine  times  the  same  effect."  1  These  results,  although 
interesting,  are  not  of  the  value  they  would  have  possessed 
if  the  tensions  of  the  odorous  vapours  had  also  at  the  same 
time  been  determined  because  the  tension  of  the  vapour 
would  influence  its  capacity  for  absorbing  radiant  heat. 

Odours  a?id  ozone. — It  is  remarkable  that  on  the  one 
hand  ozone  (condensed,  or  allotropic  oxygen,  Og),  as  pro- 
duced by  electricity,  develops  the  odours  of  the  essential 
oils,  and  on  the  other,  that  these  oils  produce  ozone  by  their 
action  on  the  oxygen  of  the  air.  Thus,  slow  oxidation  of 
oil  of  turpentine,  or  of  one  of  the  essential  oils,  produces 
ozone.  Ozone,  again,  exists  in  the  air  of  the  sea-side  when 
the  grassy  banks  are  clothed  with  wild  thyme  and  other 
scent-giving  plants,  and  it  abounds  on  the  heather-clad  hills, 
more  especially  when  the  heather  is  in  bloom.  This 
suggests  that  the  atmosphere  of  our  cities  might  be  ozonised 
and  made  more  healthy  by  the  free  use  of  odorous  substances 
like  oil  of  turpentine  or  the  perfumes. 

Odours  and  surface  tensio?i. — Some  of  the  physical 
characters  of  odorous  bodies  have  been  studied  by  Venturi, 
Prevost,  and  Lie'geois.  It  is  well  known  that  if  minute 
fragments  of  camphor  or  succinic  acid  are  placed  on  the 
surface  of  pure  water,  they  move  with  extreme  rapidity,  owing 
to  changes  in  the  surface  tension.  If  odorous  particles  are 
placed  on  a  glass  plate,  the  surface  having  been  previously 
moistened  with  water,  the  particles  at  once  fly  from  each 
other,  it  may  be  to  a  distance  of  several  inches.  This  simple 
method  constitutes  the  odoroscope  of  Prdvost.  Liegeois 
has  pointed  out  that  the  movements  of  camphor  in  water 
are  arrested  when  an  odorous  substance  is  brought  into 
contact   with   the   water.     The   odorous   oil  or   essence  at 

1  Tyndall,  Contributions  to  Molecular  Physics,  p.  99. 


The  Sense  of  Smell  9 1 

once  forms  a  pellicle  on  the  surface  of  the  water,  and  this 
pellicle  consists  of  minute  particles,  not  broader  than  from 
.001  to  .003  of  a  millimetre  (that  is,  from  0-5^^0"  t0 
■g-gJg-g-  of  an  inch).1  This  shows  how  the  dissemination 
of  odours  is  favoured  by  moist  surfaces.  Flowers  give  off 
odours  most  powerfully  after  a  shower  of  rain.  No  doubt 
also  when  the  odoriferous  substance  falls  on  the  moist 
olfactory  membrane  it  is  rapidly  disintegrated  into  ex- 
tremely minute  particles,  which  are  thus  more  readily 
brought  into  close  relation  with  the  olfactory  nerve  endings. 

These  figures,  given  by  Lidgeois,  are  probably  far  too 
high,  and  consequently  the  particles  are  much  smaller. 
Calculation  shows  that  the  thickness  of  the  layer  of  oil 
which  is  necessary  to  stop  the  movement  of  small  pieces 
of  camphor  over  a  definite  area  surface  of  water  amounts 
to  only  1.5  millionth  of  a  millimetre2  (that  is,  about  one- 
sixteen-millionth  of  an  inch). 

Special  Physiology  of  Smell. — The  air  containing  the 
odour  must  be  driven  against  the  membrane.  The  nostrils 
may  be  filled  with  an  odoriferous  substance  like  eau-de- 
cologne,  or  air  impregnated  with  sulphuretted  hydrogen, 
and  no  smell  will  be  experienced  if  no  inspiration  is  made. 
When  we  make  a  sniff,  the  air  in  the  nasal  passages  is 
rarefied,  and  as  the  odour-bearing  air  rushes  in  to  equili- 
brate the  pressure,  it  is  forcibly  driven  against  the  olfactory 
surface.  Odorous  air  passing  from  the  posterior  nares 
also  gives  rise  to  a  sensation  of  smell,  although  not  so 
intense  as  when  it  passes  in  the  normal  direction.  An 
odour  may  be  perceived  even  although  the  nostrils  are  full 
of  fluid.  Weber  stated  that  no  odour  was  noticeable  if  the 
nostrils  were  full  of  water,  but  Arensohn  has  shown  that 
this  was  because  the  water   injured  the   olfactory  surface, 

1  Li£geois,  Archiv.de  Physio logie,  1868. 

2  Lord  Rayleigh,  Proc.  Roy.  Soc,  28th  March  1890. 


92  Physiology  of  the  Senses 

and  that  if  the  water  was  replaced  by  a  weak  solution 
of  common  salt  (.07  per  cent — an  inert  fluid),  odours  were 
readily  perceived.  It  is  well  known,  also,  that  fishes  possess 
a  sense  of  smell.  Fragments  of  bait  cast  into  the  water 
soon  attract  fishes  to  a  fishing-ground,  and  that  at  depths 
into  which  little  or  no  light  can  penetrate.  The  fish  must 
smell  the  odoriferous  morsels. 

The  intensity  of  an  odour  depends  (1)  on  the  number 
of  olfactive  particles,  and  (2)  on  the  extent  of  olfactory 
surface  affected,  or,  in  other  words,  on  the  number  of 
nerve-endings  stimulated.  It  is  remarkable  that  sensations 
of  odours  are  very  evanescent.  Hence  to  maintain  the 
sensation  fresh  particles  must  be  brought  to  act  on  the 
olfactory  surface,  and  when  we  wish  to  maintain  the  sensa- 
tion experienced  in  sniffing  the  delicate  odour  of  a  flower, 
we  sniff  and  sniff  again. 

The  delicacy  of  the  sense  varies  much  in  different 
individuals  and  in  different  animals.  It  is  highly  developed 
both  in  carnivora  and  herbivora.  The  dog,  for  example, 
appears  to  depend  on  the  sense  of  smell  almost  to  as  great 
an  extent  as  on  the  sense  of  sight,  and  olfactory  impressions 
probably  are  to  him  both  more  vivid  and  more  permanent 
than  to  a  man. 

Attempts  have  been  made  to  combine  odours,  but  with- 
out success.  Thus,  if  we  fill  each  nasal  passage  with  a 
different  odour,  we  do  not  experience  a  mixture  of  two 
sensations,  but  the  odours  come  alternately,  and  we  smell 
only  one  at  a  time.  There  is  usually  a  difference  as 
regards  olfactive  sensibility  between  the  two  nasal  cavities, 
when  they  are  tested  with  the  same  odour. 

Beaunis,1  by  noting  exactly  the  moment  that  an  odour 
is  experienced  after  it  has  been  presented  to  the  nose,  has 
discovered  that  this  time  is  not  the  same  for  all  odours. 
1  Beaunis,  Recherches  expirim. ,  1884. 


The  Sense  of  Smell  93 

Some  have  greater  power  of  penetration  than  others,  the 
maximum  being  reached  by  ammonia,  and  the  minimum  by 
musk,  and  odours  analogous  to  it.  This  power  of  penetration 
is  in  the  inverse  ratio  to  the  divisibility  of  the  odorous 
substance.  He  divides  odours  into  (a)  pure  odours ;  like 
musk,  which  he  terms  scents  or  perfumes,  and  (b)  mixed 
odours,  like  that  of  peppermint,  in  which  there  is  a  com- 
bination of  odour  with  a  vague  tactile  sensibility  referred 
to  the  mucous  membrane.  To  these  we  may  add  (c) 
substances  like  acetic  acid,  that  act  at  the  same  time  on 
the  olfactory  nerves  and  on  the  tactile  nerves  of  the 
mucous  surface — the  latter  action  being  stronger  and  more 
irritating  than  in  the  case  of  &,  the  mixed  odours,  and  (d) 
substances  that  act  only  on  the  tactile  nerves,  like  carbonic 
acid. 

Mode  of  Excitation  of  the  Olfactory  Nerves. — No 
satisfactory  theory  of  smell  has  yet  been  offered.  Graham 
suggested  that  the  odorous  substance  was  probably  oxidised 
on  the  olfactory  surface,  but  this  view  was  founded  only  on  the 
observation  that  odorous  substances  are  readily  oxidisable. 
Ramsay  has  offered  the  theory  that  smell  may  be  excited 
by  vibrations — the  period  of  vibration  of  the  lighter  mole- 
cules being  too  rapid  to  affect  the  sense — then  a  number 
of  vibrations  is  reached  capable  of  exciting  the  sense 
organ,  while  beyond  an  upper  limit  the  vibrations  again 
are  not  attuned  to  the  sense  organ  and  the  odour  dis- 
appears. All  this  is  merely  speculative,  and  has  no  founda- 
tion on  experiment.  Schultze  was  inclined  to  the  view  that 
the  action  might  be  mechanical,  because  he  found  minute 
stiff  cilia  on  the  olfactory  surface,  but  this  mechanism  is 
far  too  coarse  for  the  appreciation  of  the  almost  in- 
finitesimal amount  of  odorous  substances  capable  of 
exciting  the  sense.  Stimulation  by  electricity  has  thrown 
no  light   on   the  question.      The  opening  and  closing  of  a 


94  Physiology  of  the  Senses 

continuous  current,  led  to  the  olfactory  surface  through  a 
solution  of  common  salt  at  a  temperature  of  380  C,  cause 
a  sensation  of  an  odour  like  that  of  phosphorus.  The 
action  of  odours  is  not  through  the  medium  of  the  ether, 
the  movements  of  which  account  for  the  phenomena  ot 
light.  Odours  have  to  do  with  the  grosser  forms  of  matter, 
and  all  the  evidence  is  in  favour  of  some  kind  of  chemical 
action,  the  nature  of  which,  however,  is  quite  unknown. 

Loss  of  the  sense  of  smell  is  termed  anosmia.  This  is  a 
rare  condition,  usually  congenital.  In  such  cases  all  tactile 
sensations  referred  to  the  mucous  membrane  of  the  nose,  and 
all  tactile  and  gustatory  sensations  referred  to  the  tongue, 
may  exist.  The  sense  of  smell  alone  is  absent.  Subjective 
sensations  of  odour  are  rare,  but  they  have  been  found  in 
the  insane,  and  are  due  to  excitation  of  the  part  of  the 
brain  connected  with  the  sense  of  smell. 

The  sense  of  odour,  termed  by  Kant  taste  at  a  distance, 
gives  us  information  as  to  the  quality  of  food  and  drink, 
and  more  especially  as  to  the  quality  of  the  air  we  breathe. 
Hence  we  find  the  organ  placed  at  the  opening  of  the  respira- 
tory passage  and  in  close  proximity  to  the  organs  devoted 
to  taste.  Taste  is  at  the  gateway  of  the  alimentary  canal, 
just  as  smell  is  the  sentinel  of  the  respiratory  tract,  and 
just  as  taste,  when  combined  with  smell  to  give  the  sensa- 
tion we  call  flavour,  influences  the  digestive  process,  and  is 
influenced  by  it,  so  smell  influences  the  respiratory  process. 
This  has  recently  been  shown  by  Ch.  Henry.1  He  has 
recorded  the  entrance  and  exit  of  air  by  the  nose,  with 
and  without  odours  (the  quantity  of  odoriferous  substance 
being  noted),  and  he  finds  that  the  presence  of  odours 
influences  both  the  amplitude  and  the  number  of  the 
1  Ch.  Henry,  Revue  Scientifique,  1892,  p.  73. 


The  Sense  of  Smell  95 

respiratory  movements.  Thus  the  smell  of  winter-green 
notably  increased  the  respiratory  work  ;  next  came  ylang- 
ylang  ;  and  last  rosemary.  The  breathing  of  a  fine  odour 
is  therefore  not  only  a  pleasure,  but  it  increases  the  amplitude 
of  the  respiratory  movements.  Just  as  taste  and  flavour 
influence  nutrition  by  affecting  the  digestive  process,  and  as 
the  sight  of  agreeable  or  beautiful  objects,  and  the  hearing 
of  melodious  and  harmonious  sounds,  react  on  the  body 
and  help  physiological  well-being,  so  the  odours  of  the 
country,  or  even  those  of  the  perfumer,  play  a  beneficent 
role  in  the  economy  of  life. 


THE  SENSE  OF  SIGHT 


McR 


Fig.  29. — Anteroposterior  section  through  upper 
eyelid,  X  7  d.  1,  Outer  skin — E,  epidermis  ;  C, 
corium;  Sc,  subcutaneous  tissue;  H<5,  fine  hairs; 
K,  M,  sweat  glands ;  W,  eyelash ;  W, W",  roots 
of  eyelashes ;  E^r,  reserve  hair  ;  2,  muscles  for 
closing  eye — O,  muscular  bundles  cut  trans- 
versely ;  McR,  ciliary  muscle  of  Riolanus  ;  3, 
tendon  of  muscle  elevating  the  eyelid,  mps  \  4, 
conjunctival  region  ;  tp,  tunica  propria  ;  e,  con- 
junctival epithelium  ;  at,  gland  ;  t,  tarsus  ;  in, 
Meibomian  gland  ;  a,  a',  arteries  ;  5,  corner  of 
eyelid.     (Stohr.) 


The  sense  of  sight  differs 
from  the  senses  of  taste 
and  smell  in  this  im- 
portant particular,  that 
through  it  we  seem  to  be- 
come aware  of  the  exist- 
ence of  things  which  are 
entirely  apart  from  us,  and 
have  no  direct  or  material 
link  connecting  them  with 
our  bodies.  Yet  physi- 
cists tell  us  that  in  vision 
the  eye  must  be  affected 
by  a  something  which  is 
as  certainly  material  as  a 
sapid  or  an  odorous  sub- 
stance, and  which,  per- 
meating the  universe, 
transmits  by  its  vibrations 
movements  that  affect 
the  eye,  and  give  rise  to 
the  sensation  of  light,  or 
to  the  perception  of  even 
the  most  distant  objects. 
This  medium  for  the 
transmission  of  light   is 


The  Sense  of  Sight  97 

called  the  luminiferous  ether,  and  our  eyes  are  so  constituted 
as  to  respond  to  its  vibrations  ;  changes  are  set  up  in  the 
optic  nerve  and  in  the  brain,  and  we  see. 

That  the  eye  may  be  sufficiently  sensitive  to  the  ray 
of  light,  its  sensory  surface  must  be  carefully  protected 
from  all  hurtful  influences.  Accordingly,  we  find  that  the 
eyeball,  embedded  in  soft  fat,  is  placed  in  a  socket  whose 
margins  are  formed  of  strong  bone  which  can  withstand 
heavy  blows  ;  it  is  also  protected  from  drying  by  the  action 
of  the  lachrymal  gland  which  secretes  a  watery  fluid,  and 
from  dust  and  foreign  bodies  by  the  lids  with  their  long 
eyelashes.  The  watery  fluid  which  bathes  the  eyes  passes 
away  by  two  fine  pores  at  the  inner  angles  of  the  eyelids 
into  a  passage  to  the  nose,  and  is  prevented  from  overflow- 
ing and  running  down  the  cheeks  by  an  oily  secretion 
coming  from  glands  in  the  upper  eyelid  (Fig.  29,  m)  which 
anoints  the  edges  of  the  eyelids  (Fig.  29).  Furthermore, 
the  eyebrows  protect  the  eyes  from  perspiration  trickling 
from  the  forehead.  The  eye  may  be  moved  in  various 
directions  by  muscles  which  will  be  described  later. 


I— STRUCTURE  OF  THE  EYE 

Coats  of  the  Eyeball. — The  eyeball  is  nearly  spherical 
in  shape,  but  is  slightly  elongated  from  before  backwards, 
for  the  front  part,  which  is  clear  and  transparent,  to  allow 
the  entrance  of  the  rays  of  light,  bulges  forward  somewhat 
prominently.  The  ball  is  elastic  but  firm,  and  is  enclosed 
by  a  covering  which  may  be  divided  into  three  layers,  each 
of  which  has  important  functions  to  discharge.  (For  the 
relative  position  of  the  various  parts  of  the  eyeball  see  Fig. 
30.) 

1 .   The  outermost  coating  is  composed  of  a  layer  of  firmly 

H 


98 


PJiysiology  of  the  Senses 


felted  fibrous  tissue,  which,  being  very  tough,  preserves 
the  form,  and  prevents  rupture  of  the  eyeball.  To  it  the 
muscles  that  move  the  eyeball  are  attached.  It  is  called 
the  sclerotic  (Greek,  scleros,  hard),  and  the  part  of  it  seen 
when   the   eye    is    open    is  known   as   the    "  white   of    the 


-J-7 


Fig.  30. — Diagrammatic  section  of  the  eyeball.  1,  Sclerotic ;  2,  junction  of 
sclerotic  and  cornea  ;  3,  cornea ;  4,  5,  conjunctiva ;  6,  posterior  elastic 
lamina ;  7,  junction  of  iris  with  choroid  ;  8,  canal  of  Schlemm,  a  lymph  space  ; 
9,  pigmented  tissue  uniting  sclerotic  to  choroid;  10,  choroid;  n,  12,  13, 
ciliary  processes  ;  14,  iris  touching,  but  not  connected  with  lens  posteriori}'  ; 
15,  retina  lined  by  hyaloid  membrane  ;  16,  optic  nerve  ;  17,  central  artery  of 
the  retina  ;  18,  yellow  spot  with  central  groove  ;  19,  20,  anterior  portion  of 
retina  ;  21,  junction  of  choroid  and  ciliary  processes  ;  23,  free  border  of  ciliary 
process  resting  on  anterior  suspensory  ligament  of  lens  ;  22,  canal  of  Petit ; 
24,  hyaloid  membrane  ;  25,  fibres  to  posterior  surface  of  lens  ;  26,  27,  28, 
lens  ;  29,  vitreous  humour  ;  30,  anterior  chamber  containing  aqueous  humour ; 
31,  posterior  chamber  communicating  with  30. 


eye."  In  early  childhood  the  white  of  the  eye,  being  thin, 
appears  bluish  in  tint  from  the  pigment  seen  through  it, 
while  in  old  age  it  becomes  yellowish  by  a  deposit  of  fat. 

The  clear  transparent  circular  disc  in  the  front  of  the 
eye,  the  cortzea,  is  a  modification  of  this  external  coat.      The 


The  Sense  of  Sight 


99 


fibres  of  the  cornea  are  united  by  a  cement  substance  into 
transparent  sheets  or  membranes,  which  lie  parallel  to  one 
another  like  the  coats  of  an  onion, 
but  connected  together  by  many 
intercommunicating  fibres  (Fig. 
31).  In  the  flat  spaces  between 
the  fibrous  sheets  lie  numerous 
corpuscles,  flattened,  transparent, 
and  branching  so  as  to  join  with 
one  another.  The  fibrous  sub- 
stance of  the  cornea  is  lined  in 
front  and  behind  by  a  homogeneous 
elastic  layer,  that  at  the  back  of 
the  cornea  being  the  thicker  and 
called  the  posterior  elastic  lamina 
of  Bowman,  or  the  membrane  of 
Descemet.  This  lamina  is  itself 
covered  on  its  posterior  aspect  by 
a  layer  of  flattened  cells  lying  side 
by  side  as  in  a  tesselated  pavement. 
There  are  no  blood-vessels  in  the 
cornea,  nutrition  being  effected 
through  the  branching  cells. 

The  whole  of  the  exposed  part 
of  the  eye  is  covered  with  a  trans-    FlG-  31-—  Anteroposterior  sec- 

.  -     ..  .  .  tion  of  cornea.  ^Conjunctiva; 

parent  epithelium  or  skm  called  the 


n,  nerve  sending  branches  to 
cornea  and  conjunctiva  ;  f, 
fibres  of  cornea  between  which 
are  flattened  spaces  containing 
corpuscles  ;  d,  layer  of  cells 
covering  posterior  surface  of 
cornea,  and  separated  from  the 
fibrous  part  by  the  posterior 
elastic  membrane.  (Schofield.) 

tective  covering  for  the  open  eye.  v  ' 

2.   The   middle   coat,   the   cho7'oid,  is  largely  composed 

of  blood-vessels  which  branch  frequently  in  its  outer  part, 


conjunctiva,  which  is  continuous  all 
round  with  that  lining  the  eyelids, 
and  which,  closely  adherent  to  the 
cornea,  and  more  loosely  joined  to 
the  sclerotic,  forms  a  sensitive  pro- 


IOO 


Physiology  of  the  Senses 


~5=a^^gstgggSssgp8£ffl| 


and  form  a  very  fine  network  of  capillaries  to  the  inside. 
The  blood-vessels  of  the  choroid  coat  are  known  as  the 
ciliary  arteries  and  veins.  The  veins  as  they  emerge  join 
together  in  a  stellate  fashion,  forming  groups,  the  vena 
vorticosa,  from  the  union  of  which  single  veins  pass  out- 
wards through  the  sclerotic.  The  spaces  between  the 
vessels  are  occupied  by  elastic  fibrous  tissue,  and  by  cells 
loaded  with  granules  of  very  dark  brown  pigment,  the 
whole  being  bound  together  by  cement  substance.  The 
colouring  matter  renders  the  choroid  opaque,  and  absorbs 

the  rays  of  light  pass- 
ing into  the  eye,  thus 
preventing  their  reflec- 
tion to  and  fro  in  the 
interior  of  the  eyeball, 
and  the  confused  vision 
that  would  ensue  there- 
from. 

Fig.    32. — Antero- posterior    section   through  ,         ,  ......         . 

conjunctiva  and  fore  part  of  human  cornea,  A ne  Choroid  IS  ClOSely 

X240CI.     i,  Conjunctiva ;  a,  nerve  fibres  in  united     tO     the    Sclerotic 

conjunctiva  :  s,  network  of  nerve  fibres  be-  ,  r 

tween  conjunctiva  and  cornea;  2,  anterior  bY   meanS    <*  Connective 

elastic  membrane ;  3,   substance  of  cornea  tissue,     but     JUSt     where 

with  n,  a  nerve  passing  through  it.    (Stohr.)  ,,  •,         ,•  ,,„„  :„*,. 

'  v       s        &  '   the  sclerotic  merges  into 

the  cornea  an  interesting  and  important  alteration  occurs. 
Were  the  choroid  to  line  the  cornea  as  it  does  the  sclerotic, 
light  could  not  enter  the  eye.  Accordingly  this  coloured 
layer  hangs  separate  from  the  cornea  as  a  curtain  or  ring  of 
variable  size  called  the  iris  (iris,  a  rainbow),  and  is  pierced 
by  an  aperture  known  as  the  pupil,  through  which  light 
may  enter.  The  space  between  the  iris  and  the  cornea, 
the  a,7iterior  chamber,  is  filled  with  a  watery  fluid,  the 
aqueous  humour.  The  back  of  the  iris  is  lined  with  dark 
pigment,  and  according  as  the  substance  of  the  iris  con- 
tains less  or  more  pigment,  the  eye  has  a  blue,  gray,  or 


The  Sense  of  Sight 


IOI 


brown  colour.  The  central  aperture  is  usually  black,  from 
the  pigment  absorbing  most  of  the  light  that  enters  the  eye, 
so  that  almost  none  is  reflected  out  again  ;  but  sometimes, 
as  in  albinos,  the  pigment  is  awanting,  and  then  the  pupil 
is  pink,  as  may  be  seen  in  white  rabbits.  In  many  of  the 
lower  animals  the  pupil  is  often  seen  of  a  greenish  lustre 
owing  to  partial  reflection  of  light  from  the  back  of  the  eye. 
In  herbivora  this  iridescent  gleam  is  due  to  the  arrange- 
ment of  the  fibres  to  the  outside  of  the  capillary  layer  in  a 


Fig.  33. — Meridional  section  through  ciliary  region  of  human  eye,  X  20  d.  i,  2, 
Epithelium  and  loose  connective  tissue  of  conjunctiva  ;  3,  sclerotic  ;  4  meri- 
dional, 5  radiating,  and  6  circular  fibres  of  ciliary  muscle  ;  7,  ciliary  process  ; 
8,  ciliary  part  of  retina  ;  9,  pigmentary  layer  on  the  posterior  surface  of  the 
iris;  10,  the  iris;  11,  the  posterior  elastic  lamina;  12,  the  cornea;  13,  con- 
junctiva ;  14,  canal  of  Schlemm  ;  15,  in  the  anterior  chamber  points  to  junc- 
tion of  iris  with  sclerotic.     (Stohr.) 

structure  called  the  taftetiwi,  while  in  carnivora  and  birds 
of  prey  it  is  brought  about  by  reflection  from  cells  which 
contain  minute  crystals  and  act  like  prisms. 

The  amount  of  light,  moreover,  which  enters  the  eye  is 
regulated  by  variation  in  the  size  of  the  pupil.  There  are  con- 
tractile fibres  radiating  in  the  iris  like  the  spokes  of  a  wheel, 
and  when  these  contract  the  pupil  dilates.  On  the  other 
hand,  if  too  much  light  is  entering  the  eye,  a  circular  band 
of  muscle  fibre   in  the  iris,  near  the  margin  of  the  pupil, 


102 


Physiology  of  the  Senses 


contracts,  and  the  pupil  is  lessened  in  size.  The  iris  is 
joined  to  the  sclerotic  by  muscular  as  well  as  by  connective 
tissue.  The  muscular  fibres  are  disposed,  partly  so  as  to 
radiate  from  the  junction  of  the  cornea 
and  sclerotic  to  that  of  the  iris  and 
choroid,  and  partly  to  form  a  ring 
round  the  outer  border  of  the  iris,  as 
seen  in  Fig.  33.  Together  they  form 
what  is  called  the  ciliary  muscle,  and 
this  assists  largely  in  accommodating 
the  eye  for  the  perception  of  objects 
at  different  distances.  Just  behind  the 
ciliary  muscle  lies  a  curious  modifica- 
tion of  the  choroid,  consisting  of  a  ring 
of  tooth-like  tufts  of  capillary  blood- 
vessels, bound  together  by  connective 
tissue,  and  pointing  towards  the  pupil. 
These  are  the  ciliary  processes.  The 
choroid  and  ciliary  processes  are  lined 
internally  by  a  thin  transparent  mem- 
brane, known  as  the  membrane  of 
Bruch. 

3.  The  innermost  coat,  the  retina, 
is  the  terminal  organ  of  vision,  and 
is  almost  transparent,  with  a  pinkish 
serrated  line  of  union  of  tinge,  except  at  a  point  in  the  visual 
axis  called  the  yellow  spot,  of  which 
more  anon.  The  retina  contains  the 
terminal  branches  of  the  optic  nerve, 
which,  piercing  the  sclerotic  and 
choroid  in  the  human  eye  at  a  point 
about  -jjj  of  an  inch  nearer  the  nose  than  the  antero-posterior 
axis  of  the  eye,  and  forming  an  oval  area  known  as  the  optic 
pore,   spreads  out  in  nerve   fibres   ramifying  over   all   the 


Fig.  34. — Blood-vessels  of 
the  choroid  and  iris  of 
the  human  eye  seen  from 
within,  a,  Capillary  ves- 
sels  of  the   choroid  ;   b, 


choroid  with  ciliary  pro- 
cesses ;  c,  veins  of  ciliary 
ring ;  d,  capillaries  of 
ciliary  processes  ;  e,  radi- 
ating veins  of  ciliary  part 
of  iris ;  f,  vessels  of  pupil- 
laryzoneof  iris.  (Arnold.) 


The  Sense  of  Sight 


103 


interior  of  the  eye  as  far  forward  as  the  ciliary  processes. 
These  nerve  fibres  are  the  more  transparent  as  they  are  simply 
axis  cylinders,  devoid  in  the  retina  of  the  white  substance  of 
Schwann.  They  are  supported  by  connective  tissue  which 
is  found  in  most  parts  of  the  retina  as  fibres  passing  radi- 
ally, the.  fibres  of  Midler.  The  connective  tissue  also  forms 
external  and  internal  limiting  membranes  and  a  fine  net- 
work through  the  substance  of  the  retina,  keeping  the 
various  elements  in  their  proper  places.  Small  blood- 
vessels are  also  found  in  the  inner  layers  of  the  retina. 


Pigmentary  layer  not  seen. 


Layer  of  rods  and  cones. 
External  limiting  membrane. 
Outer  nuclear  layer. 


— -J.  Outer  reticular  layer. 

6.  Inner  nuclear  layer. 

— 7.  Inner  reticular  layer. 


Fig 


I 8.  Ganglion  cell  layer. 

jjL 9,  Nerve  fibre  layer. 


,  35. — Vertical  section  of  human  retina,  X  240  d.  b,  Blood-vessel  ;  k,  conical 
base  of  radiating  sustentacula!'  fibre  of  Muller.  The  base  of  several  fibres 
uniting  gives  rise  to  the  appearance  of  an  internal  limiting  membrane.  (Stohr.) 


After  spreading  over  the  fundus  or  concavity  of  the 
retina,  the  nerve  fibrils  turn  outwards  and  become  con- 
nected with  a  set  of  ganglionic  cells  (see  Fig.  36),  from 
which,  again,  fibres  may  be  traced  outwards  for  a  certain 
distance.  These  fibres  are  believed  to  become  connected 
with  nuclei,  which  are  found  in  two  layers  to  the  outside 
of  the  ganglionic  cells,  and  from  the  outer  layer  of  nuclei 
fibres  pass  to  the  true  terminal  sensory  organ,  the  so-called 
facob's  membrane  or  layer  of  rods  and  cones.  This  layer 
lies  outside  of  and  upon  the  external  limiting  membrane. 


io4 


Physiology  of  the  Senses 


The  rods  and  cones  consist  alike  of  an  inner  and  an  outer 
part.  In  the  cones,  the  inner  part  is  thick  and  conical,  and 
exhibits  a  longitudinal  striation  (Fig.  37)  ;  in  the  rods  it  is 
thinner :  both  are  connected  with  nucleated  fibres,  internal 
to  the  outer  limiting  membrane.    The  outer  part  of  the  rods 


Fig.  36. — Diagram  showing  retinal  elements. 
Two  fibres  of  Miiller  with  expanded  bases  at 
a,  pass  outwards  as  fine  cylindrical  processes, 
giving  off  slender  lateral  twigs  (not  shown  in 
diagram)  in  the  reticular  layers  d  and  f,  and 
forming  meshworks  in  the  layers  e  and  g.  The 
spaces  of  the  meshwork  are  occupied  by  nuclei. 
The  fibres  terminate  in  the  external  limiting 
membrane  h.  Opposite  c  two  ganglionic  cells 
are  seen,  their  inner  processes  continuous  with 
optic  nerve  fibres  in  b,  their  outer  processes 
breaking  up  into  numerous  twigs  in  d.  The 
nuclei  of  the  layer  e  belong  partly  to  the  fibres 
of  Miiller,  partly  to  cells  which  send  man}' 
branching  processes  to  the  outer  and  inner  reti- 
cular layers,  and  probably  establish  functional 
continuity  between  the  ganglion  cells  and  the 
rods  and  cones.  The  nuclei  of  g  are  surrounded 
by  a  thin  layer  of  protoplasm,  and  are  connected 
externally  with  the  rods  and  cones  by  processes 
perforating  the  external  limiting  membrane, 
and  internally  by  fine  fibres  known  respectively 
as  rod  and  cone  fibres,  with  the  network  of  the 
outer  reticular  layer.  The  nuclei  connected 
with  the  rods  show  one  or  two  transverse  dark 
bands.  The  rods  and  cones  of  the  layer  i  show 
the  differentiation  into  an  outer  and  inner  limb. 
The  outer  limb  of  the  cone  is  shorter  than  that 
of  the  rods.     (Zehender.) 


is  of  a  pink  colour,  and  considerably  longer  than  that  of  the 
cones,  but  both  exhibit  a  transverse  striation,  and,  under 
the  influence  of  macerating  reagents,  tend  to  break  up  into 
highly  refractile  discs.  The  rods  are  much  more  numerous 
than  the  cones,  but  the  fore  part  of  the  retina  has  cones 


The  Sense  of  Sight 


io5 


only,  while  the  part  of  the  retina  lining  the  iris  has  neither 
rods  nor  cones.  On  the  other  hand,  in  the  yellow  spot 
above  mentioned  we  find  cones  but  no  rods.  Here,  too,  we 
find  the  layer  of  ganglion  cells  at  first 
thickened,  but  soon  thinning,  and  there 
is  formed  in  the  centre  of  the  yellow 
spot  a  short  groove  or  depression,  the 
fovea  centralis^  where  the  various  layers 
of  the  retina  above  described  disappear, 
and  we  find  only  a  layer  of  cones  with 
the  fine  terminations  of  the  nerves. 
This  spot  is  the  seat  of  most  distinct 
vision.  Outside  of,  and  in  apposition 
with,  Jacob's  membrane  lies  a  layer  of 
hexagonal  cells,  containing,  more  espe- 
cially on  their  inner  side,  a  vast  number 
of  pigment  granules  of  a  brown  colouring 
matter  called  fuscin  or  melanin.  Under 
the  action  of  light,  the  cells  send  pro- 
cesses carrying  the  pigment  inwards 
between  the  outer  segments  of  the  rods 
and  cones,  and  thus  absorb  the  rays  of 
light  after  they  have  passed  through  the 
retina.  If  the  eye  is  kept  in  darkness 
for  some  time,  these  processes  are  with- 
drawn into  the  main  bodies  of  the  cells, 
and  the  layer  of  pigmented  epithelium 
may  then  be  easily  detached  from  the 
adjoining  layer  of  the  retina  (Fig.  39). 

Contents  of  the  Eyeball. — Inside 
of,  and  closely  adherent  to,  the  retina  we  find  a  perfectly 
transparent,  highly  elastic  bag  called  the  hyaloid  membrane 
(Ziyalos,  glass),  which  might  be  compared  to  the  membrane 
lining  the  shell  of  an  egg.    This  bag  is  filled  with  a  transparent 


Fig.  37.  —  Diagram  of 
rods  and  cones,  show- 
ing faint  longitudinal 
striation  of  inner  limbs 
of  rods  and  cones,  and 
varicosities  of  the  rod- 
fibres.  (MaxSchultze.) 


io6 


Physiology  of  the  Senses 


glassy-like  jelly,  like  white  of  egg,  called  the  vitreous  humour 
(Fig.  30,  p.  98),  and  composed  of  fluid,  penetrated  in  all  direc- 
tions by  fine  fibres  and  a  few  connect- 
ive tissue  cells.      In  front,  the  hyaloid 
membrane  closely  adheres  to  the  circle 
of  ciliary  processes  but  not  to  the  iris, 
and  it  splits  into  two  layers  or  suspen- 
sory ligaments,  which  are  attached  to 
a  capsule  in  which  lies  the  crystalline 
Fig.  38.-R.ods  and  cones   lens.     The  suspensory  ligament  forms 
seen  from  without   on   a  Y'mg  called  the  zonule  of  Zinn,  and 

removal    of   pigmentary  ■,■,,■,  ■,  ,    i       ■, 

layer.  The  larger  circles   bounded  by  the  two  layers  and  the  lens 
represent  the  inner  limb   [s  a  triangular  space  containing  fluid, 

of  the  cones ;  the  smaller  .  ,,     _      ,  r       .    _,    ,.,  rTM 

central  circles,  the  outer     and    Called    the    canal  °f  PetlL         The 

limb  of  the  cones,    in  2   ligament,   it  may   be   noted,   is  much 

and  3,  the  cones  are  sur-        ,■  -,   ,        r  ,,  ,.  1      • 

rounded  by  rods.  i.Fiom    Plated  by  following   the   Convolutions 

the  yellow  spot ;  2,  from   of  the  ciliary  processes,  and  the  pos- 
or  ero  ye  ow  spot ,  3,    terjor  }ayer  [s  perforated  with  numerous 

from  middle  of  retina.  J  c 

apertures  (Fig.  30). 
The  lens  is  composed  of  fine  flattened  fibres  hexagonal 
in  cross  section,  and  with  serrated  edges  which  fit  exactly 
into  one  another,  and  are 
bound  together  by  a  kind 
of  cement  substance.    The 
fibres  run  in  an  obliquely 
meridional    direction    (see 
Fig.  41,    C),    not  forming- 
complete  semicircles  from 
pole  to  pole,  but  fixed  at 
their  ends  to  a  tri-radiate 
mass  of  cement  substance, 
whose  rays  form  angles  of 
1200  with  one  another,  and,  as  they  pass  through  the  sub- 
stance  of  the   lens,   are   rotated    like    a   wheel   in   motion 


a  ° 

Fig.  39.  —  Hexagonal  pigmented  cells 
covering  Jacob's  membrane,  a,  Surface- 
view  ;  b,  cells  seen  from  the  side,  sending 
fine  processes  between  rods  and  cones. 
The  lighter  portion  in  the  centre  of  the 
cells  in  a,  indicates  the  non-pigmented 
nucleus.     (Max  Schullze.) 


The  Sense  of  Sight 


107 


through  an  angle  of  6o°. 
The  lens,  like  the  capsule 
which  holds  it,  is  perfectly 
clear  and  transparent. 
Should  it  become  opaque, 
we  have  the  disorder  known 
as  cataract.  It  has  a  bi- 
convex form,  its  front  sur- 
face being  somewhat  more 
flattened  than  that  behind, 
but  it  is  highly  elastic,  and 
the  curves  are  constantly 
changing  as  the  eye  is 
accommodated  for  near 
and  distant  objects.  The. 
capsule  surrounding  the 
lens  is  very  thin  and  elastic, 
and,  by  the  tension  of  the 
anterior  suspensory  liga- 
ment, the  surface  of  the 
lens  is  kept  slightly  flat- 
tened. In  its  earliest  stages 
of  development,  the  lens 
is  formed  by  an  invagina- 
tion or  growth  inwards  of 
a  process  of  the  deepest 
layer  of  the  epidermis, 
which  is  cut  off  as  a  closed 
sac.  The  central  cavity  is 
obliterated  by  the  elonga- 
tion of  the  cells  at  the 
back  of  the  sac,  the  cells  in 
front  remaining  small  and 
cubical,   and   forming   the 


Fig.  40. — Lens  fibres.  A,  From  eye  of  ox 
showing  serrated  edges  ;  B,  cross  section 
of  lens  fibres  from  human  eye  ;  C,  fibres 
from  the  equatorial  region  of  the  human 
eye.  The  fibres  are  seen  edgewise  except 
in  A  and  at  C,  2.  Near  1,  nuclei  of  lens 
fibres.  (Schwalbe,  after  Kolliker  and 
Henle.) 


io8 


Physiology  of  the  Senses 


anterior  epithelium  of  the  lens.  The  lens  may  be  artificially 
broken  up  into  a  set  of  concentric  layers  (Fig.  42),  in  which 
the  fibres  run  in  a  meridional  direction,  and  the  outer  layers 
are  softer  and  more  gelatinous  than  those  towards  the  centre. 
The  lens  from  the  eye  of  a  lightly  boiled  fish  affords  con- 
venient material  for  the  study  of  the  structure  of  the  lens.  It 
appears  as  an  opaque  white  ball,  but  when  the  outer  part  is 
detached  with  a  knife  an  inner  translucent  core  is  found, 
from  which  thin  transparent  sheets  may  be  readily  peeled 


Fig.  41. — Diagram  of  arrangement  of  lens  fibres.  A  Posterior,  B  anterior, 
and  C  lateral  view,  c,  in  each  figure,  indicates  the  centre  of  the  tri-radiate 
cement  substance.  The  numbers  i  to  6  indicate  the  same  six  lens  fibres,  the 
course  they  take  being  seen  by  comparison,  of  the  three  figures.    (Schwalbe.) 


off  and  broken  up  into  fibres.  The  iris,  to  have  perfect 
mobility,  hangs  free,  not  only  of  the  cornea  in  front,  but  also 
of  the  lens  and  its  suspensory  ligament  behind,  except  in  its 
central  part  round  the  pupil,  where  it  rests  lightly  on  the 
lens.  The  space  behind  the  iris  and  in  front  of  the  lens 
and  suspensory  ligament  is  called  the  posterior  chamber. 
This  is  filled  with  fluid,  which  is  similar  to,  and  in  com- 
munication with,  the  aqueous  humour  in  the  anterior 
chamber.  We  thus  see  that  the  contents  of  the  eyeball  are 
all  transparent,  and  light  traversing  the  eye  must  pass  first 


The  Sense  of  Sight 


109 


Fig.  42. — Laminated  structure  of 
the  crystalline  lens.  The  laminae 
are  split  up  after  hardening  in 
alcohol.  i,  The  denser  central 
part ;  2,  2,  2,  concentric  outer 
layers.     (Arnold.) 


through  the  conjunctiva  and  cornea  in  front,  then  through 
the  aqueous  humour,  thereafter 
through  the  lens  with  its  capsule, 
and  finally  through  the  vitreous 
humour  and  the  hyaloid  mem- 
brane. 

The  Optic  Nerve.  —  The 
nerve  fibres  converge  from  all 
parts  of  the  retina  to  the  optic 
pore,  and  there  passing  through 
a  membrane  in  which  are  many 
fine  openings  for  their  passage, 
the  lamina  cribrosa,  they  are 
grouped  together  into  a  bundle 
forming  the  optic  nerve.  The  optic  nerve  from  each  eye 
passes  backwards,  and  entering  the  hollow  of  the  cranium 

by    a   passage   at 


the  back  of  the 
orbit,  joins  with 
its  fellow  in  a 
union  called  the 
optic  commissure. 
At  the  commis- 
sure some  of  the 
fibres  pass  directly 
upwards  into  the 
brain,   but  in  the 


human  eye  the 
most  of  the  fibres 
from  the  inner  or 

Fig.  43. — Course  of  nerve  fibres  in  posterior  part  of  nasal  half  of  each 
retina.  1,  Optic  pore  ;  2,  yellow  spot  {macula  luted);  retma  deCUSSate 
3,  fibres  to  yellow  spot.     (Schwalbe.)  ' 

or  in  other  words 
cross  over,  and  pass  backwards  to   the  half  of  the  brain 


no  Physiology  of  the  Senses 

opposite  to  the  eye  from  which  they  have  come,  while  fibres 
from  the  outer  or  temporal  (next  the  temples)  side  of  each 
retina  pass  back  to  the  brain  on  the  same  side  as  the  eye 
from  which  they  have  sprung.  Hence  it  will  be  seen  that 
almost  all  the  fibres  affected  by  rays  of  light  which  come 
from  objects  on  the  left  side  of  the  body  (a,  Fig.  44)  will 
transmit  impressions  to  the  right  side  of  the  brain,  while 
luminous  impressions  from  the  right  side  of  the  eyes  will  be 
transmitted  to  the  left  half  of  the  brain.  The  bundles  of 
nerve  fibres  continued  behind  the  optic  commissure  are 
known  as  the  optic  tracts^  and  they  pass  to  certain  ganglia 
at  the  base  of  the  brain,  from  which  again  fibres  pass  to  the 
<2V^  occipital  or  posterior  part   of 

the  cerebral  hemispheres,  the 
stimulation  of  which  gives  rise 
to  a  sensation  of  light. 

But  the   eye  is  in  connec- 
tion   with   other    nerve   fibres 
W/V  \v^\\  besides  those  of  the  optic  nerve. 

Fig.    44-Diagrammatic    representa-     We     a11     kn0W     h°W     Sensitive 
tion  of  decussation  of  fibres  of  the    the    eye    is    tO    touch,   and    how 

optic  nerves.  acutely   painful   is   any  lesion 

of  the  eyeball.  Impulses  giving  rise  to  tactile  or  painful 
sensations  are  sent  to  the  brain  through  the  medium  of 
branches  of  a  nerve  known  as  the  ophthalmic  division  of 
the  fifth  cranial,  or  great  sensory,  nerve  of  the  head,  from 
which  there  also  pass  to  the  iris  several  branches  known  as 
the  long  ciliary  nerves,  to  whose  function  reference  will 
shortly  be  made. 

Again,  the  eye,  as  a  whole,  and  certain  parts  within  the 
eye,  can  be  moved  under  the  influence  of  muscular  contrac- 
tion, and  to  effect  these  movements  we  have  the  oculo-motor 
or  third  cranial  nerve,  and  the  fourth  and  sixth  cranial 
nerves.     The  fibres  of  the  third  cranial  which  supply  the 


The  Sense  of  Sight  1 1 1 

sphincter  of  the  iris  pass  through  a  ganglion  known  as  the 
ciliary  ganglion,  where  they  meet  with  fibres  from  the 
sympathetic  system,  and  a  branch  from  the  ophthalmic 
nerve.  From  the  ganglion  a  large  number  of  twigs,  the 
short  ciliary  nerves,  pass  to  the  back  of  the  eyeball,  where, 
having  pierced  the  sclerotic  coat,  they  run  forward  between 
the  sclerotic  and  choroid  coats  to  the  ciliary  muscle,  the 
iris,  and  the  cornea.  Stimuli  pass  by  the  short  ciliary  nerves, 
as  a  result  of  which  the  pupil  may  vary  in  diameter,  or  the 
eye  be  accommodated  for  the  perception  of  objects  at  vary- 
ing distances. 

Movements  of  the  Pupil. — Various  influences  may 
cause  change  in  the  size  of  the  pupil.  The  brighter 
the  light  entering  the  eye,  the  nearer  the  object  we 
look  at,  or  the  more  we  converge  the  two  eyes,  the 
more  the  pupil  contracts.  In  certain  stages  of  poisoning 
by  opium,  tobacco,  alcohol,  chloroform,  and  physostigmin, 
in  sleep,  or  in  unconscious  states  as  during  an  epileptic 
fit,  the  pupil  may  be  contracted  to  a  mere  pin-hole 
aperture.  Dilation  of  the  pupil  occurs  when  the  light  is 
dim,  when  the  eye  is  looking  at  distant  objects,  when  respira- 
tion is  obstructed,  or  the  body  strongly  stimulated  ;  under 
the  effect  of  certain  drugs,  such  as  belladonna,  or  its  active 
principle  atropin,  by  Indian  hemp  or  hyoscyamin  ;  in  the 
later  stages  of  poisoning  by  alcohol,  chloroform,  and  other 
substances  ;  and  under  the  influence  of  mental  emotions, 
such  as  fear. 

This  change  in  size  of  the  pupil  is  an  involuntary  move- 
ment, and  goes  on  without  consciousness  upon  our  part, 
unless  we  are  directly  observing  it  in  a  mirror.  It  is  of  the 
nature  of  a  reflex  act.  The  usual  exciting  cause  of  the 
movement  is  a  variation  in  the  amount  of  light  entering  the 
eye,  and  a  consequent  variation  of  the  amount  of  stimulus 
to  the  optic  nerve.      If  the  optic  nerve   is  cut,  or   if  the 


ii2  Physiology  of  the  Senses 

centre  to  which  it  passes  in  the  brain  is  destroyed,  the 
pupil  no  longer  contracts  when  light  falls  on  the  retina, 
although  the  oculo-motor  or  short  ciliary  nerves  may  still 
be  directly  stimulated  by  electricity  or  mechanical  irritation, 
so  as  to  cause  contraction.  Moreover,  the  third  nerve  con- 
tains at  least  two  sets  of  fibres,  stimulation  of  one  of  which 
causes  contraction  of  the  pupil,  of  the  other,  movements  of 
accommodation,  and,  as  might  be  expected,  these  fibres 
originate  in  different  centres  in  the  brain.  These  centres 
are  situated  close  to  each  other  in  the  basal  ganglia,  and  on 
a  lower  level  than  the  cortical  centres  involved  in  conscious 
vision. 

The  pupil  is  caused  to  dilate  by  stimulation  of  the  sym- 
pathetic nerve  which,  coming  from  a  ganglionic  centre 
situated  in  the  neck,  and  having  entered  the  cranial  cavity, 
becomes  apposed  to  the  ophthalmic  nerve,  and  is  given  off 
to  the  eye  from  its  nasal  branch  as  the  long  ciliary  nerves. 
There  has  been  much  discussion  as  to  its  mode  of  action, 
but  apparently  it  supplies  the  dilating  muscular  fibres  of  the 
iris.  The  oculo-motor  to  the  sphincter  of  the  iris,  and 
sympathetic  to  the  dilating  fibres  of  the  iris,  would  thus 
seem  to  act  as  antagonists  to  each  other.  Moreover,  they 
seem  to  keep  up  a  constant  balancing  tonic  action,  because 
if  one  is  injured  the  other  immediately  shows  its  power. 
For  instance,  if  the  sympathetic  fibres  be  cut,  the  pupil  will 
at  once  contract,  and  vice  versa.  But  this  is  merely  a 
particular  instance  of  the  general  law  which  regulates  the 
condition  of  the  muscles  of  the  body,  so  long  as  their  nerve 
supply  is  normal  and  in  healthy  action.  Another  point  of 
interest  in  regard  to  the  human  eye  is  that  a  strong  stimulus 
to  one  eye  will  cause  contraction  of  both  pupils.  This  is 
probably  due  to  the  incomplete  decussation  of  the  optic 
nerves,  the  fibres  from  one  eye  passing,  as  we  have  seen, 
to  centres  on  both  sides  of  the  brain  ;  for  in  animals  that 


The  Sense  of  Sight  1 1 3 

have    a    complete    decussation,    and    want    the    power    of 
binocular  vision,  this  phenomenon  is  absent. 

We  should  note  in  passing  that  the  foregoing  explanation 
of  the  mechanism  of  contraction  and  dilation  of  the  pupil 
has  been  called  in  question  by  some  physiologists.  They 
deny  that  the  so-called  dilator  of  the  iris  consists  of  true 
muscular  tissue  at  all,  and  maintain  that  the  sphincter 
action  of  contraction  is  the  only  really  muscular  act.  Dilation 
is  attributed  to  elastic  recoil,  the  sphincter  being  held  to 
be  inhibited  or  thrown  out  of  action  by  stimulation  of  the 
sympathetic.  When  the  pupil  contracts,  the  elastic  radiat- 
ing fibres  are  stretched  ;  when  the  muscle  ceases  to  act, 
elasticity  comes  into  play,  and  the  pupil  dilates.  Recent 
observations  seem  to  show  that  changes  in  the  calibre  of  the 
blood-vessels  of  the  iris,  brought  about  by  nervous  action, 
are  not  the  cause  of  variations  in  the  diameter  of  the  pupil. 
The  iris  of  birds  contains  specially  developed  striated  mus- 
cular fibres,  and  a  more  careful  examination  of  such  eyes 
may  yet  throw  light  upon  this  problem. 

Drugs  may  act  either  directly  upon  the  muscles  of  the 
iris,  or  indirectly  through  the  nerve  centres.  Thus,  even  in 
an  eye  removed  from  the  body,  and  cut  off  from  all  central 
control,  atropin  will  cause  dilation,  physostigmin  contrac- 
tion of  the  pupil.  The  explanation  of  this  is  difficult,  if  we 
suppose  that  two  antagonistic  muscles  are  at  work  in  the 
eye,  for  we  would  expect  the  poison  to  act  on  each  alike, 
and  that  the  pupil  would  remain  unchanged  in  size.  On 
the  other  hand,  if  there  is  only  one  muscle  at  work,  we 
would  say  that  atropin  paralyses  it,  while  physostigmin 
excites  it  to  continuous  and  prolonged  activity.  The  varia- 
tion in  size  of  the  pupil  from  emotion,  obstructed  respira- 
tion, and  the  like,  is,  on  the  other  hand,  of  a  central 
kind — that  is  to  say,  in  such  conditions  the  activity  of 
the   central   nervous   system   is   augmented   or   diminished 

1 


ii4  Physiology  of  the  Senses 

with  a  corresponding   effect  upon   the   innervation   of  the 
eyes. 

The  observation  has  been  made  that  the  pupil  of  the  eye 
of  a  cat  isolated  after  death,  and  with  even  the  posterior 
segment  of  the  eye  cut  off,  will  slowly  contract  on  continued 
exposure  to  light.  This  appears  to  indicate  that  the  iris  is 
susceptible  to  the  action  of  light  even  without  the  presence 
of  a  nervous  mechanism. 


II— PHYSIOLOGY  OF  VISION 

The  optic  nerves  are  the  nerves  of  vision.     When  stimu- 
lated  or  injured  no  pain  is  caused,  but  only  a  luminous 
sensation  is  aroused.      Nor  are  the  nerve  fibres  sensible  to 
light,  except  in  and  through  the  retina.      Light  falling  upon 
the   exposed   optic   nerve   will   cause   no   sensation,  but   if 
the   nerve   be   now   affected   by   mechanical,  electrical,  or 
chemical    means,    a   sensation    of   a   flash   of  light   is   ex- 
perienced.      The    sensation,    however,    is     one    of    mere 
luminosity ;    it   is   not   accompanied   by  the  perception  of 
any  object.      In   order   that  an   object  may  be  perceived, 
an  image  of  it  must  be  formed  on  the  retina,  and  hence 
we   note   the   double    function   of   the   eye,    the    power   of 
responding  to  light,  due  to  the  structure  of  the  retina,  and 
the  power  of  perceiving  objects   due  to  the  nature  of  the 
transparent  media  in  front  of  the  retina. 

In  many  of  the  lower  forms  of  animals  we  find  nerves 
ending  in  coloured  spots  in  the  skin,  and  through  these  it 
may  be  the  animal  experiences  a  sensation  of  a  special  kind 
of  light ;  but,  in  the  absence  of  a  lens  or  other  refractive 
media,  images  cannot  be  formed  on  these  spots,  and  such 
animals  can  have  no  visual  perception  of  external  objects. 
It  will  conduce,  therefore,  to  a  clear  understanding  of  this 


The  Sense  of  Sight  115 

matter,  if  we  consider  briefly  the  nature  of  the  stimulus — 
light — and  the  laws  of  its  transmission  through  various 
media,  that  is  to  say,  the  laws  of  dioptrics. 

1. — Laws  of  Dioptrics 

The  Physical  Nature  of  Light. — It  was  once  held  that 
a  luminous  body  shoots  out  from  itself  minute  particles, 
which,  passing  to  the  observer's  eye,  give  rise  upon  impact 
to  the  sensation  of  light.  This  corpuscular  theory  has  now 
been  entirely  disproved,  and  it  is  generally  held  by  physicists 
that  the  undulatory  theory,  first  enunciated  by  Thomas 
Young,  affords  a  satisfactory  explanation  of  all  the  pheno- 
mena of  light.  According  to  this  view,  light,  objectively  con- 
sidered, is  simply  a  mode  of  motion  of  a  substance  called 
the  luminiferous  ether  which  pervades,  not  only  what  is 
commonly  regarded  as  space,  but  also  all  translucent  sub- 
stances. By  the  molecular  movements  of  luminous  bodies 
this  ether  is  set  vibrating  in  series  of  waves.  The  com- 
ponent particles  of  these  waves  may  be  conceived  to  move 
at  right  angles  to  the  direction  of  the  ray  of  light,  just  as 
waves  rise  and  fall  while  spreading  outwards  when  the  sur- 
face of  calm  water  has  been  agitated  by  a  stone.  Thus  a 
cork  floating  on  the  water,  traversed  by  a  wave,  oscillates 
up  and  down  nearly  at  right  angles  to  the  direction  of  the 
wave.  These  wave-like  movements  of  the  ether  impinging 
on  the  retina  set  up  in  it  changes  which  result  in  the 
sensation  of  light,  but  the  sensation  in  no  way  resembles 
its  physical  cause,  although  it  varies  with  variation  of  the 
stimulus.  The  intensity  of  the  sensation  varies  with  the 
amplitude  of  the  waves.  Large  waves  give  rise  to  a  sensa- 
tion of  bright  light,  small  waves  to  a  sensation  of  dim  light. 
Again,  the  sensation  of  colour  depends  upon  the  rapidity 
with  which  the  waves  follow  one  another.      This  rapidity, 


n6  Physiology  of  the  Senses 

though  inconceivably  great,  may  still  be  accurately  deter- 
mined. Ordinary  sunlight,  as  Newton  showed,  is  composed 
of  a  series  of  colours  blended  together,  but  yet  separable 
one  from  another,  because  each  colour  is  due  to  a  series  of 
waves  differing  in  rate  of  succession  from  the  others.  Thus 
the  waves  of  red  light  follow  each  other  at  the  rate  of  about 
435  millions  of  million  times  per  second,  while  those  of 
violet  light  succeed  each  other  at  about  764  millions  of 
million  times  per  second.  Between  these,  we  have  an 
infinite  number  of  series  of  waves,  each  giving  rise  to  a 
special  colour  sensation,  and  so  between  the  red  and  the 
violet  of  the  spectrum  we  have  a  gradation  of  colour  roughly 
described  as  orange,  gree?i,  blue,  and  indigo,  but  each  of 
these  is  itself  made  up  of  countless  shades,  which  melt  as 
gradually  and  imperceptibly  into  one  another  as  the  colours 
in  a  sunset  sky.  The  eye  is  not  sensitive  to  vibrations  of 
the  ether  succeeding  each  other  more  slowly  than  those  of 
red  light,  although  it  may  be  demonstrated  that  these  exist 
and  originate  electrical  and  thermal  phenomena ;  nor  to 
those  which  come  more  quickly,  although  these  have  marked 
chemical  activity,  and  give  rise  to  fluorescence. 

Reflection  and  Refraction. — Light  waves  are  propa- 
gated through  the  ether  at  about  190,000  miles  per  second, 
but  the  rate  varies  according  to  the  medium  through  which 
the  light  is  passing.  When  the  medium  is  homogeneous 
the  ray  passes  in  a  straight  line.  When  it  meets  a  polished 
surface  it  is  reflected  ;  and  the  angle  which  the  reflected 
ray  makes  with  a  perpendicular  to  the  surface  is  equal  to 
that  which  the  ray  meeting  the  surface,  or,  as  it  is  called, 
the  i?icident  ray,  makes  with  the  same  perpendicular. 
Further,  the  incident  ray,  the  perpendicular,  and  the 
reflected  ray  will  all  be  in  the  same  plane.  Few  surfaces, 
however,  are  so  highly  polished  as  to  conform  entirely  to 
the   above   laws.      A    certain    part   of   the   ray   is   usually 


The  Sense  of  Sight  117 

irregularly  reflected  or  scattered,  and  it  is  owing  to  this 
fact  that  objects  become  visible,  for  it  can  be  easily  under- 
stood that  if  the  rays  were  reflected  entirely  to  the  eye  we 
would  only  be  aware  of  the  luminous  body,  and  not  of  that 
which  reflects  the  light. 

When  a  ray  of  light  passing  through  one  transparent 
medium,  such  as  air,  meets  another,  such  as  water,  per- 
pendicularly, part  of  it  is  reflected  upon  itself,  and  part 
passes  on  in  the  same  straight  line  through  the  water.  If, 
on  the  other  hand,  the  ray  meets  the  surface  of  the  water 
obliquely,  the  part  which  passes  through  the  water  continues 
in  the  same  plane  as  before,  but  no  longer  passes  in  the 
same  straight  line.  It  is  bent  or  ref-acted 
out  of  its  course. 

Some  crystals  have  a  power  of  double 
refraction — Jhat  is  to  say,  the  ray  of  light      - 
entering  them  is  broken  into  two  rays,   „  „ 

J    '     l1  ig.  45. — Diagram  lllus- 

each  of  which  is  deflected  from  the  original  trating  the  law  of  the 
course;  but  as  in  explaining  the  pheno-      reflection  of  light  from 

x  a  plane  surface.     z(J, 

mena  Of  vision    we    do    not    have  tO  deal        Incident  ray ;  Or,  re- 

with  such  substances,  let  it  be  understood      flected  ray- 

that  what  we  have  to  say  with  regard  to  refraction  refers 

merely  to  simple  refraction  or  bending  of  the  ray. 

The  laws  for  si?igle  refraction  have  been  thus  stated l — 

1.  Whatever  the  obliquity  of  the  incident  ray,  the  ratio 
which  the  sine  of  the  incident  angle  bears  to  the  sine  of 
the  angle  of  refraction  is  constant  for  the  same  two  media 
but  varies  with  different  media. 

2.  The  incident  ray  and  the  refracted  ray  are  in  the 
same  plane,  which  is  perpendicular  to  the  surface  separating 
the  media. 

This  ratio  of  the  sines  of  the  incident  and  refractive 
angles  is  known  as  the  index  of  refractio?i ;  and  if  the  ray 

1  Ganot's  Physics,  p.  466. 


1 1 8  Physiology  of  the  Senses 

be  supposed  to  pass  from  a  vacuum  through  any  transparent 
substance,  this  ratio  is  known  as  the  principal  index  of 
refractioji  for  that  substance,  and  is  commonly  represented 
by  the  letter  \i. 

Knowing  the  index  of  refraction  for  any  two  media,  we 
can  calculate  the  direction  which  the  ray  of  light  will  take 
as  it  passes  through  them. 

Each  singly  refractive  substance,  then,  has  always  the 
same  bending  power  due  to  its  special  elasticity  and  con- 
sequent interference  with  the  velocity  of  the  ray  of  light. 
Water  interferes  more  than  air,  glass  than  water ;  the 
diamond  bends  the  ray  of  light  more  than  any  other  known 
substance,  or,  in  other  words,  is  the  most  refractive  sub- 
stance known. 

Effect  of  refraction  on  a  ray  passing  through  glass  with 
parallel  surfaces. — Suppose   the  ray   EF  (Fig.  46)  passing 
^  through  air  meets  obliquely  the  upper 

surface  AB  of  a  plate  of  glass  hav- 
ing parallel  surfaces.  Part  of  the 
light  will  be  reflected  in  the  direction 
FK,  part  will  pass  through  the  plate, 
but  not  in  the  original  direction  FL  ; 
it  will  be  bent  towards  XY,  the  per- 
Y  pendicular  to  the   surface,  and  will 

F?he4^ractfonmo?11rraaymoSf    take  the  Path  FG'      MeetinS  the  SUr- 

Hght.  For  description,  see  face  CD,  it  now  passes  out  into  the 
air,  where  it  immediately  regains  its 
former  velocity,  or  in  other  words,  is  bent  back  again  to  its 
former  direction,  so  that  it  now  emerges  as  GH,  not  indeed 
in  the  same  straight  line  as  before,  but  in  a  parallel  direc- 
tion to  its  former  course. 

Effect  of  refractio7i  when  light  passes  from  air  through  a 
prism. — When  light  falls  obliquely  on  the  sides  of  a  prism 
it  is  doubly  bent,  as  may  be  seen  from  the  accompanying 


A 

E\ 

/F           B 

1 
1 

\ 

C 

H 

The  Sense  of  Sight 


119 


Fig.  47. — Diagram  illustrating  re- 
fractive power  of  a  prism. 


figure.  The  ray  GH  (Fig.  47)  meeting  the  surface  AB  at 
H,  is  bent  towards  DE  in  the  direction  HK,  and  emerging 
through  the  surface  AC  is  bent 
away  from  EF  in  the  direction 
KL,  that  is  to  say,  it  is  bent 
away  from  its  original  course, 
and  deflected  towards  the  base 
of  the  prism. 

The  amount  of  deflection  de- 
pends upon  the  shape  and  material  of  the  prism,  and  on  the 
angle  at  which  the  ray  of  light  impinges  on  its  surface. 

Action  of  Lenses. — A  similar  deflecting  action  is  exer- 
cised by  lenses,  which  may  be  looked  upon  as  resembling 

two  prisms  in  apposition  by 
their  bases  or  edges.  Thus 
in  Fig.  48,  A  and  B  represent 
pairs  of  prisms  set  respec- 
tively base  to  base,  and  edge 
to  edge ;  C,  D,  and  E  are 
convex  lenses,  or,  in  other 
words,  are  thicker  at  their 
centre  than  at  their  circum- 
ference, and  would  exercise  a 
deflecting  power   upon   rays 

F,  biconcave ;  G,  plano-concave ;  H,     0f  light  similar  to  that  of  A  ; 
convexo-concave  lens. 

F,  G,  and  H  are  concave 
lenses,  being  thinner  at  their  centres  than  their  circum- 
ference, and  would  deflect  rays  of  light  in  the  same  way 
as  B.  The  biconvex  lens  is  of  most  interest  for  our  present 
purpose,  for,  like  the  transparent  media  of  the  eye,  it  has 
the  property  of  condensing  or  focussing  rays  of  light. 

The  common  burning-glass  or  biconvex  lens  has,  as  a  rule, 
spherical  surfaces.  If  AB  (Fig.  49)  represent  a  biconvex 
lens,  and  the  line  CF  its  principal  axis,  i.e.  the  straight  line 


Fig.  48. — Diagram  showing  comparison 
of  lenses  to  prisms  set  base  to  base 
or  edge  to  edge.  C,  Biconvex ;  D, 
plano-convex  ;    E,    concavo-convex  ; 


120 


Physiology  of  the  Senses 


through  the  centre  of  curvature  of  its  two  surfaces,  all  rays 
parallel  to  CF  meeting  the  surface  ADB,  will  be  brought 
to  a  focus  at  very  nearly  the  point  F,  which  is  called  the 

principal  focus  ;  and,  con- 
versely, rays  spreading 
from  F  will  pass  through 
the  lens,  and  emerge  in  a 
parallel  direction. 

If  rays  diverge  from  a 
point  /  (Fig.  50)  in  the 
axis  of  the  lens  outside 
of  the  principal  focus,  they  will  be  brought  to  a  focus  at  a  point 
f  on  the  other  side  of  the  lens  known  as  its  conjugate  focus. 


Fig.  49. — Diagram  illustrating  course  taken 
by  parallel  rays  of  light  refracted  by  bicon- 
vex lens. 


Fig.  50. — Diagram  illustrating  the  law  of  conjugate  foci. 

If,  as  in  Fig.  51,  the  rays  diverged  from  f  to  the  inside 
of  F,  they  would  still  diverge  on  the  other  side  of  the  lens  ; 


Fig.  51. — Diagram  illustrating  position  of  virtual  focus. 

but  now  if  produced  backwards,  would  form  a  virtual  focus 
at/'. 

Formation  of  Images  by  Biconvex  Lenses. — Any  ob- 
ject at  which  we  look  may  be  regarded  as  made  up  of  an 
aggregation  of  points,  each  of  which  sends  a  pencil  of 
rays  of  light  to  the  eye,  and  the  main  value  of  the  lens  for 
purposes  of  vision  is  its  power  of  forming  images  of  objects 


The  Sense  of  Sight 


121 


by  combining  again  the  scattered  rays.  Thus  all  the  rays 
from  A  falling  on  CD  (Fig.  52)  may  be  collected  at  the 
point  A',  all  the  rays  from  B  at  B',  and  rays  from  all 
intervening  points  of  AB  will  meet  at  points  along  the  line 
A'B'j  and  thus  an  image  of  AB  is  formed,  but  upside 
down  or  inverted. 

The  size  and  position  of  the  image  depend  on  the 
position  of  the  object  with  regard  to  the  principal  focus  of 
the  lens,  and  can  be  calculated  by  simple  .mathematical 
formulae.  In  Fig.  52,  for  example,  the  rays  from  the 
point  A  of  the  object  AB  may  be  supposed  to  be  brought 
to  a  focus  by  the  lens  CD  at  the  point  A'.      Those  from  B 


Fig.  52. — Formation  of  an  image  by  a  biconvex  lens. 

at  B',  and  all  intermediate  points  in  AB,  at  corresponding 
points  in  A'B'. 


We  are  now  in  a  position  to  understand  why  a  lens  is 
required  for  vision.  Were  light  simply  to  pass  through  the 
pupil  and  fall  on  the  retina  without  refraction,  from  each 
point  in  the  field  of  vision  a  cone  or  pencil  of  rays  would  pass 
to  the  retina  and  form  a  circle  of  light  upon  it,  and  these 
circles  overlapping  one  another,  as  in  Fig.  53,  would  simply 
give  a  sense  of  diffused  light,  and  not  the  perception  of  each 
point  separate  one  from  another.  But  suppose  the  pupil 
were  narrowed  to  the  finest  point,  so  that  only  one  ray  of 
light  would  pass   in  from  each  point  of  the   object,  as  in 


122 


Physiology  of  the  Senses 


Fig.  54,  the  amount  of  light  admitted  would  be  so 
infinitesimally  small  as  to  be  unable  to  affect  the  retina. 

In  avoiding  overlapping, 
the  amount  of  light 
admitted  has  become 
infinitely  little ;  or,  in 
other  words,  as  the 
pupil  diminished  in  size 

Fig.  53.— Diagram  showing  overlapping  of  rays    the  object  would  appear 
in  the  absence  of  a  lens.  . 

dimmer  and  dimmer, 
until  it  ceased  to  be  seen  altogether,  for  the  amount  of  the 
stimulus  would  be  too  small  to  excite  the  sensation  of  vision. 
But  the  refractive  media  of  the  eye  acting  like  a  lens  con- 
dense the  rays  which  have  entered  the  pupil  so  as  to  form 
an  image  which,  in  the  normal  eye,  falls  upon  the  retina  ; 
and  each  point  of  the  image,  being  the  focus  or  meeting- 
point  of  a  vast  number  of  rays  coming  from  the  correspond- 
ing point  of  the  object,  is 
sufficiently  bright  to  stimu- 
late the  retina  to  action. 

We    may    easily    prove 
that  such  is  the  case.      If 

an    eye    removed     from    its  Fig.  54--For  explanation,  see  text. 

socket  be  stripped  posteriorly  of  the  sclerotic  coat,  an  inverted 
image  of  the  field  of  view  will  be  seen  on  the  retina  ;  but 
if  the  lens  or  other  part  of  the  refractive  media,  be  removed, 
the  image  will  become  blurred  or  disappear  altogether. 

There  are,  however,  two  defects  in  ordinary  spherical 
lenses  which,  as  they  affect  the  eye,  deserve  our  notice. 

Spherical  Aberration. — Any  one  who  has  attempted 
with  a  burning-glass  to  focus  the  rays  of  the  sun  upon  a 
sheet  of  paper  must  have  noticed  that  the  circle  of  light,  at 
first  large  and  dim,  gets  smaller  and  brighter  for  a  time 


The  Sense  of  Sight  123 

and  then  enlarges  again,  but  the  image  of  the  sun  thus 
formed  is  never  reduced  to  a  mathematical  point.  This 
is  due  to  what  is  called  the  spherical  aberration  of  the  lens, 
and  a  glance  at  Fig.  5  5  will  enable  us  to  understand  it. 
The  ray  of  light  CD,  which  passes  through  the  centre  of 
the  lens  AB,  in  Fig.  55,  is  not  refracted  at  all,  but  passes 
on  in  a  straight  line.  Rays  near  CD,  such  as  E,  E,  are 
slightly  bent  and  intersect  CD  at  a  considerable  distance 
from  the  lens.  Rays  meeting  the  surface  of  the  lens  at 
points  nearer  its  circumference  than  E,  E,  such  as  G,  G, 
or  K,  K,  are  more  refracted,  and  intersect  CD  at  points 
nearer  the  lens.  Thus,  as  we  pass  towards  the  circumfer- 
ence, the  rays  are  more  and  more  refracted,  and  do  not 


B 

Fig.  55. — Spherical  aberration. 

meet  all  at  one  point.  Accordingly,  when  we  interpose  a 
screen  in  the  path  of  the  rays,  while  a  few  may  be 
accurately  brought  to  a  focus  upon  the  screen,  the  great 
majority  are  either  still  converging  or  now  diverging,  and 
they  form  concentric  rings  of  light  which  blend  with  one 
another,  or  diffusion  circles,  as  they  are  sometimes  called, 
and  these  blur  the  image  formed  by  the  accurately  focussed 
rays. 

By  interposing  a  diaphragm,  with  a  central  aperture, 
the  outer  rays  may  be  cut  off  and  only  those  rays  which 
pass  near  the  centre  will  be  brought  to  a  focus,  and  thus  the 
image  will  be  made  sharper.  If  the  central  part  of  the 
lens  be  more  refrangible  than  the  circumference,  a  similar 


124 


Physiology  of  the  Senses 


result  will  be  obtained,  for  rays  passing  through  the  former 
will  be  more  refracted,  and  thus  be  brought  to  a  focus 
nearer  those  that  have  passed  through  the  circumference. 
Such  a  provision  as  this  exists  in  the  human  eye,  the  centre 
of  the  crystalline  lens  being  more  refrangible  than  the  outer 
parts. 

Chromatic  Aberration. — The  other  defect  in  ordinary 
simple  lenses  is  that  when  sunlight  passes  through  them, 
owing  to  the  different  refrangibilities  of  the  various  coloured 
rays  which  go  to  make  up  white  light,  the  sun's  ray  is 
broken  up  into  its  component  parts,  and  some  of  these  are 
bent  more  than  others.  This  separation  of  the  coloured 
rays  is  known   as  dispe7'sion.     The   red   rays   being   least 

b 


a  h 

Fig.  56. — Chromatic  aberration. 

refrangible  are  less  refracted  than  the  orange,  the  orange 
than  the  yellow,  and  so  on,  the  violet  rays  being  most 
refracted  of  all.  Thus,  if  rays  pass  through  the  lens  AB 
(Fig.  56),  we  may  suppose  the  red  rays  to  intersect  the  main 
axis  at  R,  the  violet  at  V.  If  a  screen  be  interposed  in 
the  position  aa,  there  will  be  a  coloured  circular  spectrum 
having  the  red  to  the  outside  and  the  violet  to  the  inside  ; 
but  if  the  screen  be  placed  at  bb,  the  violet  rays  will  now 
be  outermost  and  the  red  rays  to  the  inside.  It  was 
formerly  supposed  that  the  dispersive  power  of  all  bodies 
was  alike,  but  it  is  now  known  that  this  is  not  so  ;  and  by 
combining  lenses  of  opposing  action  it  has  been  found 
possible  to  do  away,  to  a  very  great  extent,  with  the  disper- 


The  Sense  of  Sight  125 

sion  of  the  light,  although  it  is  still  refracted.  Such  a  lens 
is  usually  composed  of  a  concave  flint-glass  (A,  Fig.  57),  and 
a  biconvex  crown-glass  lens  (B,  Fig.  57),  and 
is  said  to  be  achromatic,  or  in  other  words, 
not  colour-producing. 

Optical  Properties  of  a  System  of  Lenses. 

Fig.  57. — Achro- 

— If  the   rays  of   light    emanating   from   an      mat;c  iens<   a, 
object  pass  through  a  series  of  lenses,  differ-      Plano-concave 

lens     of     flint- 

ing  in  shape  and  refractive  power,  but  having  glass .  B)  bi_ 
their  centres  in  one  axis,  the  position  and  convex  lens  of 
size  of  the  resulting  image  might  be  found  by 
calculating  and  combining  the  effect  of  each  lens  in  turn. 
This  would,  however,  frequently  lead  to  very  elaborate  cal- 
culations, and  the  researches  of  Gauss,  Mcebius,  Listing,  and 
others  have  shown  that  for  any  system  of  centred  spherical 
surfaces  there  exist  six  points  known  as  cardinal  points, 
through  four  of  which  pass  planes  perpendicular  to  the  axis, 
and  that  if  the  position  of  these  has  been  determined  the 
direction  of  all  rays  of  light  through  the  system  may  be 
readily  traced.  The  cardinal  points  are  the  first  and 
second  focal,  first  and  second  principal,  and  first  and  second 
nodal  points,  and  the  pla?ies  pass  through  the  two  first 
pairs. 

1.  The  first  focal  point  is  so  placed  with  regard  to  the 
system  that  all  rays  passing  from  it  through  the  system, 
emerge  in  a  direction  parallel  to  the  axis  of  the  system, 
while  all  rays  parallel  to  the  axis  before  entering  the 
system  are,  having  passed  through  it,  gathered  at  the 
second  principal  focal  point.  This  also  holds  good  for  all 
points  in  the  planes  through  the  foci  perpendicular  to  the 
axis. 

2.  The  first  and  second  principal  points  are  so  situated 
that  in  the  planes  passing  through  them  perpendicular  to 
the   axis — the  principal  planes — there   are    correspondent 


126 


Physiology  of  the  Senses 


points  on  the  same  side  of,  and  at  the  same  distance  from, 
the  principal  axis  of  the  system,  through  which  the  refracted 
rays  must  pass.  Thus  each  principal  plane  is  the  optical 
image  of  the  other. 

3.  The  first  and  second  nodal  points  are  such  that  all 
rays  which  before  being  refracted  pass  through  one  of 
them,  seem  after  refraction  to  emerge  from  the  other  and 
in  a  direction  parallel  to  what  they  had  at  first. 

4.  The  first  principal  focal  length  is  the  distance 
between  the  first  focal  point  and  the  first  principal  point. 

5.  The  second  principal  focal  length  is  the  distance 
between  the  second  focal  point  and  the  second  principal 
point. 

6.  The  principal  points  are  at  the  same  distance  from 

B     C 


Fig.  58. — Diagram  illustrating  course  of  ray  through  a  dioptric  system. 

each  other  as  the  nodal  points,  and  the  distance  between 
the  first  focus  and  the  first  nodal  point  is  equal  to  that 
between  the  second  focus  and  the  second  principal  point. 
Then  the  distance  between  the  first  principal  and  first 
nodal  points  equals  the  difference  between  the  first  and 
second  principal  focal  lengths. 

Given  the  cardinal  points  we  may,  then,  trace  the 
course  of  a  ray  through  the  system  or  calculate  the  position 
and  size  of  the  image  of  an  object. 

Supposelin  Fig.  58  Y^v  PjPg,  NXN2  represent  re- 
spectively the  first  and  second  focal,  principal,  and  nodal 
points.  Any  ray  AB  from  the  first  focal  plane  incident  upon 
the  first  principal  plane  passes  parallel  to  the  main  axis  to 


The  Sense  of  Sight 


127 


C,  and  thence  in  a  direction  parallel  to  AN,  the  line  joining 
A  to  the  first  nodal  point. 

To  find  the  position  of  the  image  of  any  point  A,  we  must 
trace  the  course  of  at  least  two  rays  from  the  point  through 
the  system  till  they  meet.      Thus,  in  Fig.  59,  with  the  same 


Fig.  59. — Image  of  a  point. 


letters  as  above,  the  ray  AB  parallel  to  the  main  axis 
passes  through  C,  and  thence  through  the  second  focus  F2, 
while  from  N2  emerges  a  ray  parallel  to  ANj  which  meets 
CF2  produced,  at  A1. 


2. — The  Dioptric  System  of  the  Eye 

It  was  stated  (p.  109)  that  light,  before  falling  on  the 
retina,  passes  through  a  series  of  transparent  refractive  sub- 
stances, viz.  the  cornea,  aqueous  humour,  crystalline  lens, 
and  vitreous  humour,  and,  with  certain  exceptions,  which 
will  be  pointed  out  later,  the  eye  may  practically  be  con- 
sidered as  composed  of  a  centred  system,  composed  of  a 
convex  refractive  surface,  the  cornea,  and  of  a  biconvex  lens, 
the  crystalline  lens.  The  cornea  in  reality  has  a  double 
surface,  but  the  outer  and  inner  surfaces  are  so  nearly 
parallel  that  the  two  may  be  regarded  as  one ;  and 
although  the  lens  differs  much  in  the  refrangibility  of  its 
different  parts,  its  action  as  a  whole  may  be  taken  as  that 
of  a  homogeneous  substance.  The  surface  which  exercises 
the  greatest  refractive  influence  is  the  anterior  surface  of 
the   cornea,    since    the    refractive    powers   of  air   and   the 


128  Physiology  of  the  Senses 

substance  of  the  cornea  differ  in  a  marked  degree.  On  the 
other  hand,  the  aqueous  humour  approximates  so  nearly  in  re- 
fractive power  to  the  substance  of  the  cornea  that  the  refrac- 
tion in  it  may  be  neglected  ;  and,  again,  the  refractive  power 
of  the  vitreous  is  the  same  as  that  of  the  aqueous  humour. 

Many  careful  investigations  have  been  made  as  to  the 
form  of  the  various  refracting  surfaces  of  the  eye,  their 
relative  distances  from  one  another,  and  of  the  refractive 
powers  of  the  different  media  concerned,  and  while  it  is 
found  that  the  eyes  of  different  persons,  and  even  of  the 
same  person,  differ  to  a  considerable  extent  in  all  these 
respects,  yet  certain  measurements  have  been  obtained 
which  may  be  regarded  as  representing  those  of  an  average 
normal  eye.  These  being  known,  we  can  determine  the 
position  of  the  cardinal  points,  and  thus  calculate  the 
course  of  rays  of  light  in  the  eye.  The  following  figures 
represent  the  latest  and  most  accurate  determinations  : 1 — 

Index  of  refraction  of  the  air     .  .  .         .  .  n—\. 

Index   of  refraction   of    the    aqueous   humour    and 

vitreous  body      .......  «'=  1-3365. 

Total  index  of  refraction  of  the  crystalline         .  .  n"=  1-4371. 

Radius  of  curvature  of  the  cornea      ....   r=7»829mm. 

Radius  of  the  anterior  surface  of  the  crystalline  lens    r'  =  10mm. 
Radius  of  the  posterior  surface  of  the  crystalline  lens  r"  —  6mm. 
Distance  from  the  anterior  surface  of  the  cornea  to 

the  anterior  surface  of  the  crystalline  =  3-6mm. 

Distance  from  the  anterior  surface  of  the  cornea  to 

the  posterior  surface  of  the  crystalline  .  .       =  7 -2mm. 

Hence,  thickness  of  the  crystalline  .         .  .  .     e  —  3 -6mm. 

From  these  data,  the  following  results  have  been 
calculated  : — 

(A)  Focal  Points. 
I.   Surface  of  cornea. 

First  focal  distance  U  =  —^ —  =  — — >  -      =  23 -266mm. 
ri-i     1-3365-1 

1  Landolt,    The  Refraction  and  Accommodation  of  the  Eye,  p.  79. 


The  Sense  of  Sight  129 

Second  focal  distance/0"=-?^-  = l'3$6S  x  7'829= 31.095mm. 

w'-i         i-3365-i 

II.  Anterior  surface  of  crystalline. 

First  focal  distance/^  =  -^L=     I-3365  x  *°     -132.853mm. 

Second  focal distance/i"=  -^-,  =     I<4371  X  *°    =i42-8-nmm. 
ri'-ri     i-437i-i-3365 

III.  Posterior  surface  of  the  crystalline. 

First  focal  distance  f{  =  „= '^AL =  8^7ii7mm. 

ri-n"    i-3365-i-437i 

Second  focal  distance  72"=— — — -  = 'ALA =  70-7 11 3mm. 

»'-«"     I-3365-I-437I 


(B)  Principal  Points. 

I.   The  principal  points  of  the  cornea  coincide  with  its  summit. 

II.  The  first  and  second  principal  points  of  the  crystalline  are  at 
a  distance  of  2'i2597mm.  and  1.2756mm.  respectively  from 
the  anterior  and  posterior  surfaces  of  the  lens. 

(C)  The  Nodal  Points  of  the  crystalline  coincide  with  its 
principal  points. 

Hence  it  is  deduced  that — 

(1)  The  first  principal  focus  of  the  eye  is  situated  13 -7451  mm. 

in  front  of  the  cornea.  The  remaining  cardinal  points 
of  the  eye  are  behind  the  cornea,  and  measuring  from 
its  anterior  surfaces  lie  at  the  following  distances. 

(2)  The  second  principal  focus  of  the  eye  is  situated  22.8237mm. 

behind  the  cornea.  This  distance,  in  other  words,  is 
the  length  in  the  normal  eye  between  the  cornea  and 
the  retina. 

(3)  The  first  principal  point,  1 -7532mm. 

(4)  The  second  principal  point,  2-noimm. 

(5)  The  first  nodal  point,  6-9685mm. 

(6)  The  second  nodal  point,  7.3254mm. 

From  Fig.   60  (p.    130)  it  will  be  seen  that  the  prin- 
cipal  points  lie  in  the   anterior   chamber,    the  first   nodal 

K 


13° 


Physiology  of  the  Senses 


point  in  the  lens,  the  second  nodal  point  slightly  behind  it, 
the  first  principal  focus  in  front  of  the  eye,  and  the 
second  principal  focus  at  the  posterior  surface  of  the 
retina.  The  diagram  represents  what  has  been  called 
by  Listing  the  schematic  eye.  By  its  aid  we  may  easily 
trace  the  course  of  all  rays  of  light  entering  the  eye.  The 
principal  points  and  the  nodal  points  are  seen  to  be 
respectively    very   near   each   other,    and   if   each   pair   be 


Fig.  60. — Schematic  eye.  A,  Anterior  surface  of  cornea  ;  i//, «//',  first  and  second 
principal  focus  ;  H',  H",  first  and  second  principal  points  ;  K',  K",  first  and 
second  nodal  points  ;  F.c,  fovea  centralis  of  yellow  spot.     (Landolt.) 

regarded  as  combined  into  one  point,  we  simplify  the  con- 
ception of  the  eye  very  much,  reducing  it  to  a  system 
having  a  single  spherical  surface  separating  the  air  from 
the  more  refractive  media  of  the  eye  behind.  The  prin- 
cipal point  is  then  at  the  surface,  and  the  nodal  point  at 
the  centre  of  the  sphere,  the  focal  points  being  situated  as 
before.  Such  a  conception  is  known  as  the  reduced  eye  of 
Listing. 


The  Sense  of  Sight  131 

3. — Anomalies  in  the  Eye  as  an  Optical 
Instrument 

While  we  may  then  form  a  conception  of  a  mathematically 
correct  eye,  it  must  be  borne  in  mind  that  all  eyes  present 
certain  variations  from  the  ideal  form. 

1.  Thus  the  various  refractive  surfaces  are  not,  as  a 
rule,  centred  so  that  the  optic  axis  or  line  joining  their 
centres  coincides  with  the  line  of  vision,  that  is  to  say, 
with  the  line  from  the  point  viewed  to  the  fovea  ce7itralis 
of  the  retina.  The  angle  of  the  one  axis  to  the  other, 
where  they  meet  at  the  nodal  point,  may  be  as  great  as 
12°.  This  divergence  of  the  optic  from  the  visual  axis  is 
represented  in  Fig.  60,  where  it  will  be  noted  that  the 
posterior  end  of  the  optic  axis  does  not  go  to  the  fovea 
centralis. 

2.  Again,  the  centre  around  which  the  eye  rotates  is 
usually  in  the  optic  and  not  the  visual  axis,  and,  con- 
sequently, the  line  joining  the  point  viewed  with  the  centre 
of  rotation  of  the  eye,  or,  as  it  is  called,  the  line  of  regard, 
does  not  usually  coincide  with  the  line  of  vision. 

3.  Further,  we  have  seen  (p.  124)  that  in  ordinary  lenses, 
white  light  is  broken  up  into  coloured  rays  which  are 
not  focussed  at  the  same  point,  and  we  saw  how  we  can 
correct  this  by  combining  lenses  of  different  forms  and 
dispersive  powers.  Similarly,  in  the  eye,  the  rays  of  light 
are  broken  up  into  their  constituent  colours,  but  this  is 
done  only  to  a  very  slight  extent,  and  does  not  interfere 
with  ordinary  vision.  In  fact,  its  existence  can  only  be 
determined  by  careful  experimentation.  When  we  look  at 
red  letters  on  a  violet  ground,  the  eye  is  soon  fatigued  by 
the  effort  to  focus  both  colours  on  the  retina  at  once,  and 
we  experience  an  unpleasant  jarring  effect ;  or  in  looking 
at  a  violet  flame  which  gives  forth  red  and  blue  rays,  we 


132 


Physiology  of  the  Senses 


may  either  see  a  red  flame  with  a  blue  halo,  or  a  blue 
flame  with  a  red  halo,  according  as  the  eye  is  accommodated 
for  red  or  blue.  This  may  be  called  the  defect  of  chromatic 
aberration. 

4.  The  blurring  of  the  image  caused  by  spherical 
aberration  (p.  122)  is  almost  entirely  corrected  in  the  eye  by 
the  varying  refractive  power  of  the  media,  especially  of  the 


Fig.  61. — Astigmatism.  The  lens  ACDEF  has  greater  refractive  power  in  the 
plane  ACD  than  in  the  plane  AEF  ;  rays  in  the  vertical  plane  ACD  will  be 
brought  to  a  focus  at  the  point  G,  while  those  in  the  horizontal  plane  AEF 
are  still  converging  to  meet  at  the  point  B.  If  a  screen  be  held  at  the  point  G, 
a  horizontal  line  of  light  aa!  will  be  seen  ;  if  at  the  point  B,  a  perpendicular 
line  be  ;  and  if  at  intermediate  points,  ellipses  of  varying  shapes  as  above. 

lens,  by  the  influence  of  the  iris  in  cutting  off  the  outer  rays, 
and  by  the  shape  of  the  refracting  surfaces,  which  are  not 
spherical,  but  of  forms  known  as  ellipsoids  of  revolution, 
that  is  to  say,  surfaces  formed  by  the  rotation  of  an  ellipse 
upon  one  of  its  axes. 

5.  Astig?natism.  —  But     these     surfaces,    while    better 
adapted  for  vision  than  spherical  surfaces,  are  themselves 


The  Sense  of  Sight 


*33 


usually  somewhat  irregular  in  this  respect,  that  their  curva- 
tures vary  in  different  planes.  In  the  vertical  meridian  the 
curve  is  in  most  eyes  more  convex  than  that  in  the  horizontal  ; 
and,  as  a  result,  rays  in  a  vertical  plane  are  brought  to 
a  focus  nearer  than  those  passing  through  the  horizontal. 
Thus  all  rays  diverging  from  a  point  cannot  be  exactly 
recombined  to  a  point  after  passing  through  the  eye, 
and  a  line  is  seen  either  in  a  horizontal  or  vertical  direc- 
tion according  to  the  position  of  the  retina,  or  there  is 
a  diffusion  ellipse  for  intermediate  positions.  Hence  the 
name  astigmatism  given  by  Whewell,  from  a,  without,  and 
stigma,  a  point.  That  most  eyes 
are  more  or  less  astigmatic  is 
shown  by  the  fact  that  to  almost 
every  man  the  fixed  stars  seem  to 
twinkle  or  send  out  scintillations 
radiating  from  a  centre.  Were 
our  eyes  perfect,  the  stars  would 
appear  as  luminous  points,  not 
"  star-shaped."  Similarly,  in  look- 
ing at  the  bars  of  a  window,  the 
astigmatic  eye  cannot  see  both 
vertical  and  horizontal  bars  at  the 

same  time  with  the  same  distinctness,  one  or  other  must 
be  blurred  by  diffusion  circles.  Astigmatism  may  be  regular, 
as  above  described,  or  irregular,  the  latter  more  especially 
being  due  to  irregularities  of  the  lens,  while  the  former  arises 
most  commonly  from  the  shape  of  the  cornea.  The  effect 
is  so  slight  in  most  eyes  as  to  go  unobserved,  but  it  may  be 
so  great  as  to  require  the  use  of  a  lens  consisting  of  the 
longitudinal  segment  of  a  cylinder,  in  which  the  convexity 
is  greater  in  one  plane  than  in  another  to  compensate  for 
the  deficient  convexity  of  curvature  in  one  meridian  as 
compared  with  the  other  (Fig.  62). 


Fig.  62. — Cylindrical  lens  to  cor- 
rect astigmatism  in  the  eye. 
Rays  in  two  horizontal  planes 
are  brought  to  a  focus,  but  do 
not  approximate  in  a  vertical 
direction. 


J34 


Physiology  of  the  Senses 


4- — Adjustment  of  the  Eye  for  different  Distances 

When  parallel  rays,  such  as  come,  for  example,  from  a 
star,  fall  upon  the  normal  eye  in  a  state  of  rest  they  are  brought 
to  a  focus  on  the  retina.  If,  however,  the  rays  emanate  from 
a  point  within  a  distance  of  about  65  metres  (71  yards),  they 
are  sensibly  divergent,  and  can  only  be  brought  to  a  focus 
upon  the  retina  by  an  effort,  and  the  nearer  the  object  viewed 
is  to  the  eye  the  greater  must  be  the  effort,  until  at  last  the 
eye  becomes  unable  to  gather  the  rays  to  a  point  at  the 
retina,  and  the  object  is  no  longer  distinctly  seen.  If, 
shutting  one  eye,  we  hold  up  a  pencil  in  line  with  an  object 
at  some  distance  it  will  be  found  that  both  cannot  be  seen 
distinctly  at  the  same  time.      If  we  see  the  distant  object 


Fig.  63. — For  description,  see  text. 

distinctly  the  outline  of  the  pencil  is  blurred,  and  vice  versa. 
The  eye  has  the  power  of  adjusting  itself  so  that  all  rays 
from  beyond  a  certain  near  point  may  be  focussed  on  the 
retina.  Thus  if  the  rays  from  a  point  ft  (Fig.  63)  are  re- 
fracted so  to  meet  at  r  the  retina,/  will  be  seen  distinctly,  but 
if  the  point  ft  be  now  moved  to  the  point  ft',  unless  the  eye 
be  adjusted  for  the  change,  the  rays  from  ft'  will  be  focussed 
behind  the  retina,  and  the  point  p  would  be  seen  indis- 
tinctly. Now,  there  are  two  ways  in  which  this  adjustment 
might  be  effected.  The  length  of  the  eye  might  be  varied 
to  meet  the  varying  distance  of  the  focal  point,  just  as  a 
photographer  moves  the  sensitive  plate  of  his  camera  back- 
wards or  forwards  to  bring  it  into  focus.      But,  as  a  matter 


The  Sense  of  Sight 


i35 


of  fact,  another  process  takes  place  in  the  eye.  The  retina 
is  not  moved  backwards  or  forwards,  but  the  refractive 
power  of  the  crystalline  lens  is  changed  by  an  alteration  of 
its  thickness.  The  more  curved  the  surfaces  of  a  lens  are,  the 
greater  is  its  refractive  power.  Now,  when  we  look  at  distant 
objects,  and  no  effort  at  accommodation  is  required,  the 
anterior  surface  of  the  lens  is  kept  flattened  by  the  pressure 
of  its  capsule  and  by  the  elastic  pull  upon  it  of  the  anterior 
suspensory  ligament — an  elastic  pull  which  involves  no 
muscular  strain,  and  consequently  no  fatigue.  But  when 
we  wish  to  look  at  a  near  object,  the  ciliary  muscle  (see 
p.  10 1)  contracting  pulls  forward  the  suspensory  ligament 


Fig.  64. — Mechanism  of  accommodation.  A,  The  lens  during  accommodation 
with  its  anterior  surface  advanced ;  B,  the  lens  at  rest ;  C,  position  of  the 
ciliary  muscle ;  D,  the  vitreous  humour ;  a,  the  anterior  elastic  lamina  of 
cornea  ;  c,  corneal  substance  proper  ;  b.  posterior  elastic  lamina. 

and  diminishes  its  circle  of  attachment,  its  tension  is 
lessened,  the  pull  on  the  capsule  of  the  lens  diminishes,  and 
the  lens,  by  its  own  elasticity,  assumes  a  more  spherical 
shape,  its  anterior  surface  moving  forward,  and  its  power 
of  converging  rays  being  increased.  The  nearer  the  object, 
the  greater  the  effort  required,  and  when  long  sustained  the 
greater  is  the  fatigue  experienced.  As  a  rule,  however,  we 
are  unconscious  of  the  effort,  although,  as  will  be  seen,  the 
feeling  gives  us  valuable  aid  in  judgment  as  to  the  distances 
of  objects.  The  accompanying  'diagram  (Fig.  64)  repre- 
sents the  change,  the  right  side  B  showing  the  condition  of 
rest,  the  left  A  the  state  when  the  eye  is  adjusted  for  near 


136 


Physiology  of  the  Senses 


Fig.  65.- 


-Reflected  images  in  the  eye.    A,  for  distant ; 
B,  for  near  vision. 


sight.      The  change^in  the  curvature  of  the  anterior  surface 

of  the    lens    may   be    demonstrated   as    follows  :   Let    the 

observer  in  a  dark 
room,  looking  at  the 
side  of  the  eye  to 
be  examined,  note 
the  reflections  of  a 
candle  flame  held 
to  the  other  side, 
and  in  front  of  the 
eye  observed.   Two 

bright  points  can  be  readily  seen — one  the  reflection  of  the 

flame  from   the  surface   of  the  cornea,   and  one  from  the 

anterior  surface  of  the  lens — and, 

with  care,  a  third,  much  fainter, 

from  the  posterior  surface  of  the 

lens.       When    the   person   whose 

eye  is  being  examined  is  directed 

to  look  as  at  an  object  at  a  great 

distance,  the  three  points  of  light 

will  have  the  position  shown  in  A 

(Fig.  65) ;  and  now  on  adjusting 

the  eye   so  as  to   see  an  object 

close  at  hand  the  middle  point  of 

light  moves  forward,  nearer  to  the 

corneal    reflection,    and   becomes 

smaller  as  in  B.      This  is  due  to 

the  bulging  forward  of  the  lens, 

and  the  consequent  reflection  of 

the  light  from   a   surface   nearer 

the  cornea,  and  more  curved  than 

before.      The  experiment  can  be 

readily  performed  in  daylight  by 

means  of  the  phakoscope  invented  by  von  Helmholtz,  which 


Fig.  66. — Phakoscope.  The  ob- 
server looking  through  the  aper- 
ture a  sees  images  of  the  slits 
bb'  reflected  from  the  observed 
eye  situated  at  the  distant  side 
of  the  phakoscope,  and  accom- 
modated first  for  distance,  and 
second  for  near  vision,  the  re- 
gard in  the  latter  case  being 
fixed  on  the  needle-point  in  the 
window  c. 


The  Sense  of  Sight 


i37 


consists  of  a  darkened  box  applied  to  the  eye,  with  aper- 
tures at  convenient  positions  for  the  light,  for  the  eyes  of 
the  experimenter  and  of  the  person  observed,  and  with  an 
opening  through  which  the  eye  to  be  observed  may  look. 
Careful  measurements  of  the  sizes  of  the  reflected  images 
have  shown  that  the  image  on  the  anterior  surface  of  the 
lens  becomes  smaller  when  we  look  at  a  near  object,  another 
proof  that  the  lens  becomes  more  convex  anteriorly.  There 
is  also  a  slight  increase  in  the  posterior  convexity  of  the  lens. 
The  Near  Point  of  Vision. — The  range  of  accommoda- 
tion is  limited.  It  begins  for  objects  at  about  65  metres 
(71  yards)  from  the  eye,  and  for  normal  eyes  reaches  to 


x       y        2 
Fig.  67. — Schemer's  experiment.     For  description,  see  text. 

within  20  centimetres  (8  inches).  The  position  of  the  near 
point  of  any  eye  may  be  readily  determined  by  the  classical 
experiment  of  Scheiner.  It  is  performed  as  follows  :  In  a 
thick  card  make  two  small  holes  with  a  needle  at  a  distance 
not  greater  than  the  diameter  of  the  pupil,  and  holding  the 
paper  closely  to  the  eye  look  at  the  needle  through  the 
holes.  If  the  needle  be  held  4  or  5  inches  from  the  eye 
two  points  will  be  seen,  but  as  the  needle  is  gradually 
moved  farther  away  the  two  points  will  be  seen  to  coalesce 
into  one  point,  and  they  do  so  at  the  near  point  of  vision, 
namely,  8  inches  from  the  eye. 

The  meaning  of  this  will  be  understood  from  the  diagram 
in  Fig.  67.      If  the  needle  is  at  the  nearest  point  at  which 


138    .  Physiology  of  the  Senses 

the  rays  coming  from  it  to  all  parts  of  the  pupil  can  be 
collected  to  one  point  on  the  retina,  the  cones  of  rays 
passing  through  the  apertures  will  be  collected  at  r,  and 
we  see  the  needle  single,  but  on  bringing  the  needle 
nearer  to  the  eye  we  are  unable  to  adjust  the  eye  for  the 
divergent  rays,  and  it  is  as  if  the  retina  were  situated  at 
22,  and  two  points  a  and  b  will  be  seen  ;  but  as  these  are 
due  to  circles  of  diffusion  and  not  to  rays  brought  to  a  point, 
the  image  on  the  retina  is  blurred,  and  not  so  bright  as 
before,  owing  to  the  lessened  quantity  of  light  admitted  by 
the  single  hole.  As  the  image  is  projected  outward  through 
the  nodal  point  N,  the  image  of  b  will  be  seen  in  the  line  bb', 
and  that  of  a  in  the  line  aa',  in  other  words,  the  real  point 
seems  to  be  split  into  two,  one  on  each  side  of  the  true 
position. 

The  distances  given  above  for  the  far  and  near  points 
are  those  for  a  normal  eye  at  rest,  in  which  the  optic  axis 
is  of  such  a  length  that  parallel  rays  are  brought  to  a  focus 
on  the  yellow  spot  (Fig.  68,  1).  Such  an  eye  is  called  emme- 
tropic, or  an  eye  in  measure.  But  many  eyes  are  not  so 
adapted  ;  they  have  the  retina  either  before  or  behind  the 
focal  point,  and  are  then  said  to  be  ametropic,  or  not  in 
measure.  The  axis  may  be  too  long,  and  parallel  rays  are 
focussed  before  they  reach  the  retina  (Fig.  68,  4),  as  in  the 
short-sighted,  myopic,  or  hypometropic  eye  ;  or  the  axis  may 
be  too  short,  as  in  the  long-sighted  or  hypermetropic  eye, 
and  the  rays  are  brought  to  a  focus  behind  the  retina  (Fig. 
68,  3).  A  short-sighted  person,  who  desires  to  see  distant 
objects,  wears  spectacles  with  concave  lenses  to  make  the 
parallel  rays  diverge,  so  that  on  passing  through  the  eye  they 
will  be  brought  to  a  focus  farther  back  than  usual,  and  so  upon 
the  retina  ;  while  in  viewing  near  objects,  as  in  reading,  the 
book  is  held  nearer  the  eyes  to  give  greater  divergence  to 
the  rays.      The   long-sighted    person,   on   the   other  hand, 


The  Sense  of  Sight  139 

wears  convex  lenses,  so  that  the  rays  may  be  brought  more 
quickly  to  a   focus,  and  in   reading  he  holds  the  book  at 


Fig.  68. — i,  Emmetropic  eye  ;  2,  normal  eye  accommodated  for  near  vision  by 
increased  curvature  of  the  anterior  surface  of  the  lens  ;  3,  hypermetropic 
eye  ;  4,  myopic  eye. 

arm's  length  for  a  similar  reason.  Further,  an  eye  of 
normal  length  may  gradually  lose  its  power  of  adjustment  for 
near  objects,  a  condition  common  in  old  age,  and  we  have 


140 


Physiology  of  the  Senses 


what  is  known  as  the  presbyopic  eye.  In  the  eye  of  an  old 
person  the  parts  are  deficient  in  elasticity,  and  the  fibres  of 
the  ciliary  muscle  are  probably  less  powerful  than  in  early 
life.  The  anterior  surface  of  the  lens  cannot  therefore 
become  sufficiently  convex  for  objects  viewed  a  little  beyond 
the  near  point  of  distinct  vision.  In  other  words,  the  near 
point  in  a  presbyopic  eye  is  farther  back  than  normal,  and 
hence,  in  reading,  the  head  is  thrown  back  and  the  news- 
paper held  as  far  away  as  possible.  In  this  case,  too, 
convex  lenses  are  used  to  compensate  for  the  lost  power  of 
adjustment  for  near  objects. 

Irradiation. — A   minor    result    of  defective   power   of 


Fig.  69. — Irradiation. 

accommodation  is  to  be  found  in  the  phenomenon  known 
as  irradiation.  When  we  look  at  a  bright  object  on  a  dark 
ground  it  seems  larger  than  when  a  dark  object  of  similar 
size  is  seen  on  a  light  ground.  People  dressed  in  white 
look  larger  than  when  in  black.  Note  also  the  two  small 
squares  in  Fig.  69.  The  white  seems  larger  than  the 
black,  although  they  are  of  exactly  the  same  size.  This  is 
probably  due  in  part  to  the  formation  of  circles  of  diffusion, 
the  more  powerful  stimulus  of  the  rays  from  the  white 
surface  annulling  the  less  intense  rays  from  the  dark  border. 
An  interesting  example  of  this  is  the  effect  produced  on  the 
eye   by  the  glowing   filament   of  the   electric   lamp.      The 


The  Sense  of  Sight  141 

filament  may  form  a  loop,  but  this  is  not  seen  when  the  full 
light  of  the  lamp  meets  the  eye.  We  see  only  a  brilliant 
light.  But  if  we  cut  off  some  of  the  rays  by  the  intervention 
of  a  plate  of  smoked  glass,  or  by  winking  the  eyes  rapidly, 
the  filament  is  distinctly  seen,  although  apparently  broader 
than  it  really  is  on  account  of  the  intensity  of  its  luminosity. 

Entoptic  Phenomena. — In  describing  the  effects  of 
refraction  on  the  rays  passing  through  the  eye,  we  have 
hitherto  spoken  as  if  the  '  transmitting  media  were 
perfectly  transparent  in  all  parts.  It  has  now  to  be 
observed  that  in  almost  every  eye  there  are  small  opaque 
bodies  which  intercept  the  light  as  it  enters,  and  throw 
shadows  on  the  retina.  These  shadows  projected  out- 
wards give  the  impression  of  rounded  or  filamentous  bodies 
floating  in  space.  They  may  be  well  observed  by  looking 
with  half-shut  eyes  at  a  white  cloud,  when  they  will  be  seen 
floating  away  and  eluding  our  efforts  to  keep  them  at  rest. 
They  have  been  called  on  this  account  muscce  volitantes, 
and  their  fleeting  character  is  due  to  the  fact  that  they  are 
not  as  a  rule  directly  in  the  line  of  distinct  vision,  and  in 
our  attempt  to  gain  a  direct  view  of  them  we  move  the 
eye  and  with  it  the  substance  which  gives  rise  to  the 
appearances.  The  opaque  particles  may  be  either  in  front 
of  the  retina  or  in  the  retina  itself,  and  one  of  the  latter 
phenomena,  namely,  the  shadows  of  the  retinal  vessels,  is 
of  especial  interest,  not  only  from  its  peculiar  appearance, 
but  also  from  the  proof  which  it  affords  that  the  layer  of 
rods  and  cones  is  the  part  of  the  retina  sensitive  to  light. 
It  may  be  studied  as  follows.  In  a  dark  room  cast  a 
bright  ray  of  light  sideways  upon  the  cornea.  This  pene- 
trating to  the  retina  forms  there  a  luminous  image  which 
itself  is  reflected  to  other  parts  of  the  interior  of  the  retina. 
One  of  these  reflected  rays  may  in  its  course  impinge  upon 


142 


Physiology  of  the  Senses 


a  retinal  vessel  which  casts  its  shadow  on  the  outer  corre- 
sponding part  of  the  retina.  The  part  of  the  retina  upon 
which  the  shadow  falls,  refers  this  outwards  through  the 
nodal  point  of  the  eye.  The  path  described  is  traced  in 
Fig.  70,  A.  The  ray  b  passing  to  c'  and  reflected  thence, 
falls  on  a  vessel  x  in  the  retina,  and  a  shadow  is  cast  at  d 
which  is  referred  outwards  in  the  direction  da! .  If  now 
the  source  of  light  be  moved  to  b'  the  ray  will  pass  to  c, 
be    reflected   in    the   direction   cd\  and   intercepted   at    ,r, 


Fig.  70. — Diagram  to  illustrate  the  formation  of  Purkinje's  figures. 

with  consequently  a  shadow  on  d'  which  is  referred  out- 
wards in  the  direction  d'a.  If  the  ray  of  light  cannot  enter 
the  eye  by  the  pupil,  but  merely  passes  through  the  sclerotic, 
we  will  have  the  result  depicted  in  Fig.  70,  B.  A  ray  of  light 
entering  at  a'  is  intercepted  by  a  vessel  c,  and  the  shadow 
at  a'  is  projected  outwards  to  A.  If  we  now  move  the  source 
of  light  so  that  the  ray  enters  at  b",  the  shadow  of  c  will 
be  formed  at  b'  and  projected  outwards  to  B",  or,  in  other 
words,  we  will  see  a  dark  line  apparently  moving  from  A 
to  B". 


The  Sense  of  Sight  143 

As  a  result,  then,  of  this  play  of  light  and  shadow,  there 
is  seen  dimly  outlined  on  a  darkly  luminous  ground,  and 
moving  as  the  light  moves,  an  arborescent  figure,  the 
shadow  of  the  arteries  and  veins  of  the  retina.  We  do 
not  see  this  under  ordinary  circumstances,  because  light 
enters  the  pupil  from  all  parts  of  the  field  of  vision,  and 
no  distinct  shadows  are  cast  upon  the  retina.  H.  Miiller 
has  proved,  by  a  study  of  the  mathematical  conditions  of 
this  phenomenon,  that  the  shadows  of  the  vessels  must  fall 
upon  the  layer  of  rods  and  cones  in  order  to  give  the 
result  obtained,  or,  in  other  words,  that  light  must  penetrate 
the  various  internal  layers  of  the  retina  and  affect  the  outer 
layer  before  it  can  give  rise  to  a  sensation  of  luminosity. 

Examination  of  the  Interior  of  the  Eye. — The  pupil 
of  a  normal  eye  is  black  in  appearance,  and  we  cannot 
study  by  unaided  vision  the  interior  of  another  eye  in  situ. 
Does  the  eye  merely  absorb  rays  and  reflect  none  out- 
wards ?  Von  Helmholtz,  who  has  done  so  much  in 
advancing  the  science  of  physiological  optics,  was  the  first 
to  show  that  the  eye  does  reflect  rays  outwards,  and  that 
with  proper  arrangements  we  may  cause  the  eye  to  reflect 
so  much  light  that  its  interior  can  be  easily  examined. 

When  walking  in  the  street  we  can  scarcely  see  into 
the  interior  of  houses  through  the  windows,  because  the 
amount  of  light  emerging  from  within  is  so  much  less  than 
the  diffused  light  outside,  and  the  difficulty  is  increased  by 
the  reflection  of  light  from  the  glass.  But  we  can  see  into 
the  room  better  if  the  window  is  open,  or  if  the  room  is 
lit  up  within.  Similarly  with  the  eye,  the  light  entering 
is  partially  reflected  outwards  by  the  retina,  but  most  of 
it  is  absorbed ;  and,  further,  the  part  reflected  emerges  in 
the  same  path  as  it  entered,  and  by  the  refracting  action 
of  the  eye  is  brought  to  a  focus  at  the  original  luminous 
point.      If,  then,  we  place  a  light  between  our  eye  and  that 


144 


Physiology  of  the  Senses 


of  the  person  observed  we  cannot  see  into  the  other's  eye, 
because  the  emergent  rays  are  focussed  at  the  flame  and 
do  not  form  an  image  in  our  eyes.  If  we  bring  our  eye 
near  to  the  observed  eye,  our  own  head  intercepts  the  rays 
from  without,  and  we  cannot  see  the  interior.  But  if  a 
light  (in  Fig.  71)  be  placed  to  one  side  of  the  observed 
eye  C,  and  its  rays  reflected  into  the  eye  by  a  piece  of 
transparent  glass,  or  better  still,  by  a  small  concave  mirror 
with  a  central  aperture,  these  rays  will  illuminate  the  eye. 
Then  part  of  the  rays  again  reflected  outwards  will  pass 

through  the  glass  to  meet  and 
form  an  image  at  #,  but 
being  intercepted  by  the  ob- 
server's eye  B,  the  image  is 
formed  on  his  retina,  and  thus 
the  interior  of  the  eye  C  may 
be  examined.  It  will  be  seen 
that  this  only  holds  good  if 
both  eyes  are  emmetropic.  If 
one  eye  be  myopic,  the  other 
must  be  hypermetropic  to  a 
corresponding  degree,  and 
in  the  ophthalmoscope — the 
instrument  invented  by  von  Helmholtz  for  the  examination 
of  the  interior  of  the  eye — there  are  usually  convex  and 
concave  lenses  by  which  the  observer  is  able  to  counteract 
the  effect  of  any  degree  of  ametropia  in  the  observed  eye. 
In  other  words,  if  the  observer's  eye  be  emmetropic,  the 
nature  and  curvature  of  the  lens  which  must  be  interposed 
give  an  indication  of  the  nature  and  amount  of  the  ametropia 
of  the  observed  eye.  Thus,  by  the  ophthalmoscope,  we 
can  see  the  interior  of  the  eye,  examine  all  its  parts,  and 
judge  if  it  be  healthy,  while  at  the  same  time  we  determine 
any  short  or  long-sightedness  present. 


Fig.  71. — Principle  of  the  ophthalmo- 
scope.    (Fick.) 


The  Sense  of  Sight  145 

The  retina  presents  to  the  observer's  eye  the  appearance 
of  a  red-coloured  concave  disk,  with  a  whitish  oval  spot  to 
its  inner  side  where  the  optic  nerve  enters,  from  which  are 
seen  branching  the  retinal  vessels,  the  veins  being  darker 
in  colour  than  the  arteries,-  and  in  the  visual  axis  lies  the 
yellow  spot  already  described.  The  vessels  of  the  fovea 
centralis  are  so  fine  as  to  be  invisible  to  the  naked  eye, 
but  they  form  a  very  close  and  fine  network  at  this  part  of 
the  eye.  The  retina  being  concave,  all  images  formed  on 
it  larger  than  points  must  share  in  its  concavity.  This, 
however,  is  an  advantage,  for  if  the  retina  were  flat,  all 
the  outer  parts  of  any  image  formed  upon  it,  not  being 
exactly  focussed,  would  be  distorted,  as  on  the  plate  of  a 
camera,  but  on  account  of  the  retina's  concavity  each  part 
of  the  image  is  focussed  in  its  proper  position,  and  distor- 
tion and  blurring  thus  largely  avoided. 

While  this  is  so,  it  is  always  to  be  borne  in  mind  that 
although  the  whole  posterior  part  of  the  retina  may  have 
formed  upon  it  a  fairly  clear  and  distinct  image  of  all  the 
parts  of  the  visual  field,  and  although  by  an  act  of  will  we 
may  without  moving  our  eyes  pay  attention  to  the  outlying 
parts,  still  the  only  part  of  the  retinal  image  which  gives 
rise  to  distinct  vision  is  that  formed  upon  the  nerve  termin- 
ations in  the  central  depression  in  the  yellow  spot.  In 
other  words,  if  the  rays  of  light  from  an  object  at  which  we 
are  looking  converge  towards  the  optic  centre,  so  as  to  form 
an  open  angle,  and  then  diverging,  are  brought  to  a  focus  on 
the  retina,  to  form  a  large  image,  we  will  not  be  able  to  see 
the  whole  object  distinctly  without  moving  the  eye,  so  that 
a  series  of  images  of  different  parts  of  the  object  is  formed 
consecutively  upon  the  area  of  acute  vision. 

The  Visual  Angle.  —  The  angle  formed  by  the  rays 
from  the  extreme  limits  of  the  object  of  vision  at  their 
point    of    convergence    (the    nodal    point)    in    the    eye    is 

L 


146 


Physiology  of  the  Senses 


known  as  the  visual  angle,  and  the  visual  angle  which 
any  object  subtends  depends  upon  the  size  and  the  distance 
of  the  object  from  the  eye.  A  small  visual  angle  is  there- 
fore a  condition  of  distinct  vision.  But  there  is  a  limit  to 
this,  for  with  most  people,  if  the  visual  angle  subtended  by 
the  object  be  less  than  60",  the.  area  of  the  retina  stimulated 
will  be  so  small  that  all  separate  points  in  the  object  seem  to 
be  fused  into  one  in  the  mental  picture  obtained  by  the  retinal 
stimulation.  Some  carefully -trained  observers  with  acute 
eyes  may  possibly  distinguish  from  one  another  as  separate 
points  the  ends  of  a  line  which  subtends  an  angle  of  only 

5  o",  the  image  of  which 
in  the  average  nor- 
mal eye  would  have 
a  length  of  -00365 
mm.  or  3-65  /x.1  The 
diameter  of  a  retinal 
cone  is   3*2  ix.  but  as 

Fig.   72. — Visual  angles.     The   objects  c,  a,  e,  '  ' 

though  of  different  sizes,  subtend    the   same  the  COneS  do  not  preSS 

visual  angle,  being  at  different  distances  from  against      one      another 

the  eye.  ° 

each  cone  corresponds 
to  an  area  having  a  diameter  of  4  fx.  If  the  image  is  so  small 
as  to  fall  entirely  upon  one  cone  all  points  in  it  will  be  fused 
together,  but  it  is  conceivable  that  an  image  not  more  than 
1  /jl  in  length  might  stimulate  adjacent  sides  of  two  cones. 
In  such  a  case,  however,  there  must  be  a  mental  fusion  of 
the  effect,  for  images  of  less  diameter  than  3-65  \x  are  always 
seen  as  one,  and  not  more  than  one,  point,  at  least  so  far  as 
observations  have  yet  been  made.  It  matters  not  how 
large   the  object  may  be,  if  it  is  only  far  enough  away  to 

1  The  Greek  letter  jx  is  used  to  denote  the  thousandth  of  a  mm., 
and  is  the  unit  of  measurement  for  objects  of  microscopical  size.  A 
mm.  —  ^g-  of  an  inch  :  hence  a  micromi Hi metre,  1  M  =  -a'sooir  °f  an  mcn> 
and  3.65  /u^^Yo  of  an  inch. 


The  Sense  of  Sight  147 

subtend  the  angle  of  50"  it  must  appear  as  a  point.      The 
fixed  stars  we  know  to  be  vast  suns,  but  they  appear  to  us 
as   mere  points  of  light  because  their  dis-       ^_^^ 
tance  is  so  great  that  they  subtend  a  very      r    *jL       j 
small  visual  angle.      Nay  more,  many  stars      V_y  V. — / 
long   supposed  to  be  single    have,    by   the   FlG'  73;— Diagram 

o  rr  o  ?        j  showing  now  an 

aid  of  powerful    telescopes,  been  shown  to      image  smaller 
be  double,  triple,  quadruple,  or  even   mul-      than    *Je    dia" 

'  meter  of  a  cone 

tiple  stars,  at  vast  distances  from  one  another,      may  affectone,  or 
and   yet    appearing  as   one    to    the    naked      more  than  one' 

cone  at  the  same 
eye.  time.  The  image 

For  distinctness  of  vision  the  eye  must      affecting  two 

cones  is  actually 

have  what  we  may  call  resolving  power,  the      smaller  than  that 
power  of  keeping  each  point  of  the  image      affecting   one 
clear  and  distinct  from  its  neighbour,  and 
this  power  we  have  said  is  greatest  in  the  yellow  spot.     For 
example,  the  two  dots  below  are  easily  recognised  as  two,  if 


we  look  directly  at  them  ;  but  if  we  look  a  little  to  one  side, 
the  two  will  apparently  fuse  into  one  whenever  their  images 
are  displaced  from  the  yellow  spot  and  fall  upon  an  adjoining 
part  of  the  retina.  By  means  of  a  pencil  we  can  map  out  on 
the  page  an  area  of  irregularly  oval  shape  corresponding  to 
the  oval  shape  of  the  yellow  spot,  an  area  in  which  the  two 
dots  are  seen  as  double  and  not  fused.  The  greater  the 
distance  between  the  dots,  the  further,  cceteris  ftaribtis^  from 
the  yellow  spot  of  the  retina  may  they  be  distinguished  as 
such,  or  in  other  words,  the  further  we  pass  on  the  retina 
from  the  yellow  spot  the  less  resolving  power  does  the 
retina  possess. 

We  have  indicated  above  the  shortest  distance  between 
two  points  which  will  allow  of  their  being  seen  as  two.  A 
much  smaller  area  of  stimulation  of  the  retina  is  sufficient 


148  Physiology  of  the  Senses 

to  give  rise  to  distinct  vision.  A  luminous  point  or  line 
may  be  seen  as  such  which  gives  rise  to  an  image  that 
occupies  only  a  very  small  part  of  a  cone  or  row  of  cones. 
An  object  -04  mm.  (g^-g-  of  an  inch)  in  breadth  at  a 
distance  of  25  mm.  (1  inch)  from  the  eye  gives  a  retinal 
image  of  about  -002  mm.  (T2To~o  °^  an  incn)  *n  breadth, 
and  yet  it  is  distinctly  visible.  This  is,  however,  by  no 
means  the  minimum  visibile.  Objects  as  small  as  the 
^—i—g.-  of  an  inch  in  diameter  (about  one -tenth  of  the 
length  of  a  wave  of  light)  maybe  seen  with  the  highest 
powers  of  the  modern  microscope.  It  is  hardly  necessary 
to  state  that  even  these  minute  objects  are  many  thousands 
of  times  larger  than  the  molecules  or  atoms  of  matter  dealt 
with  by  the  physicist. 

The  Size  of  the  Retinal  Image. — The  size  of  the  image 
of  an  object  upon  the  retina  may  be  calculated  by  a  simple 
formula  if  we  know  the  size  of  the  object,  its  distance  from 
the  nodal  point,  and  that  of  the  nodal  point  from  the  retina. 
In  the  average  normal  human  eye  the  distance  of  the  nodal 
point  from  the  retina  is  approximately  16  mm.,  and  from 
the  nodal  point  to  the  anterior  surface  of  the  cornea  7  mm. 
Let  the  size  of  the  object  be  represented  by  X,  its  distance 
in  mm.  from  the  anterior  surface  of  the  cornea  by  ft, 
and    therefore    from    the     nodal    point    by  ft  +  7.     Then 

Xx  16 

fi+7  :  16  :  :X  :x,   the  size   of  the   image;    or  x=  . 

Suppose,  for  example,  the  object  looked  at  be  the  page 
of  this  book,  which  is  nearly  182  mm.  long,  and  that  the 
book  is  held  half  a  metre  (500  mm.)  from  the  eye.  Then 
the    length    of    the    retinal    image    of    the    page    will    be 

l82  X  l6  ,-i,i 

x= =  W  mm.,  or  a  little  less  than  one  quarter 

500  +  7 
of  an  inch.     Again  the  length  of  any  small  letter  on  the 
page   is   approximately    1    mm.,   hence    the    height    of   its 


The  Sense  of  Sight  149 

retinal   image,    the    book    being    held    as    before,    will    be 

= =  -03  mm.,  or  about  ^hr  of  an  inch.      The 

500  +  7      507         J  '  80° 

above-mentioned  formula,  however,  gives  only  the  length 
of  any  diameter  of  the  object  in  a  plane  perpendicular  to 
the  line  of  vision.  To  calculate  the  area  of  the  image  on 
the  retina  we  have  only  to  remember  that  the  area  of  the 
image  is  to  the  area  of  the  visual  field  occupied  by  the 
object  as  the  square  of  the  distance  of  the  image  from  the 
nodal  point  is  to  the  square  of  the  distance  from  the  nodal 
point  to  the  object.  The  flat  retinal  image  cannot,  of 
course,  correspond  in  area  to  the  superficial  area  of  a  solid 
body,  but  only  to  a  part  of  the  field  of  vision  cut  off  by  a 
plane  projection  of  the  object  upon  it.  It  is  as  if  the  visual 
field  were  a  canvas,  every  point  of  which  is  filled  by  the 
representation  of  some  external  object,  and  the  retinal 
image  is  an  exact  copy,  but  reduced  in  size,  of  nature's 
picture.  The  full  moon  and  a  ball  held  in  the  hand  give 
alike  a  flat  circular  retinal  image,  but  in  the  "mind's  eye" 
each  may  be  seen  as  a  sphere,  although  the  play  of  light 
and  shade  on  the  nearer  object  renders  the  effort  of  imagin- 
ation easier  with  it  than  in  the  case  of  the  more  remote. 

The  Blind  Spot. — It  is  interesting  to  note  that  near 
the  area  of  greatest  sensitivity  to  light  we  have  a  spot  in 
the  retina  which  is  devoid  of  rods  and  cones,  and  hence  is 
quite  unaffected  by  images  formed  upon  it.  This  is  the 
optic  papilla^  or  place  of  entrance  of  the  optic  nerve,  and  its 
diameter  being  about  1-8  mm.,  it  subtends  a  visual  angle 
of  about  6  degrees.  Lines  drawn  from  the  border  of  the 
optic  pore  to  the  nodal  point  and  produced  outwards  will 
enclose  a  flattened  cone  whose  base  is  contained  within  the 
visual  field,  and  within  which  all  objects  will  be  invisible  to 
the  unmoving  eye.  Suppose,  for  example,  the  left  eye 
being  shut,  the  right  eye  be  fixed  upon  the  cross  in  Fig.  74. 


150  Physiology  of  the  Senses 

When  the  book  is  held  at  arm's  length,  both  cross  and 
round  spot  will  be  visible ;  but  if  the  book  be  approximated 
to  about  8  inches  from  the  eye,  the  regard  being  kept 
steadily  upon  the  cross,  the  round  spot  will  at  first  dis- 
appear, but  as  the  book  is  brought  still  nearer  both  cross  and 
spot  will  again  be  seen.  It  may  also  be  noted  in  this  ex- 
periment, that  there  is  no  consciousness  of  a  break  of 
continuity  in  the  visual  field,  no  sensation  as  we  might 
imagine  there  would  be  of  darkness  ;  to  put  it  generally, 
there  being  no  stimulation,  there  is  not  consciousness  of  a 
lack,  but  a  lack  of  consciousness. 

An  attempt  has  been  made  to  determine  the  rate  of 
decrease  of  acuteness  of  vision  as  we  pass  outwards  from 
the  yellow  spot,  and  Volkmann  holds  that  it  diminishes 
proportionally  to  the  square  of  the  distance  from  the  yellow 


Fig.  74. 

spot,  but  the  determination  is,  in  its  nature,  very  hard  to 
make,  and  much  depends  on  individual  peculiarities. 

Action  of  Light  on  Retina. — This  will  be  the  more 
readily  understood  if  we  consider  for  a  moment  the  intimate 
nature  of  the  action  of  light  on  the  retina.  It  has  been 
experimentally  observed  that  if  the  eye  be  kept  in  the  dark 
for  a  time,  and  if  light  then  be  allowed  to  fall  full  on  the 
retina,  there  is  a  change  in  its  electrical  condition.  This 
phenomenon  is  evidence  of  change  in  the  condition  of 
the  molecules  of  the  sensitive  parts  of  the  retina,  which 
might  be  merely  a  change  of  rate  of  molecular  motion 
such  as  results  from  a  variation  of  temperature  of  a  body, 
or  it  might  be  due  to  a  chemical  transformation  or 
rearrangement  of  the  molecules  so  as  to  form  new  chemical 
substances. 


The  Sense  of  Sight  1 5 1 

That  the  latter  is  more  probably  the  case  may  be  held 
upon  various  grounds.  If  heat  rays  be  substituted  for 
light  in  the  foregoing  experiment  the  electrical  change  will 
not  occur.  Further,  it  has  been  observed  in  the  frog's  eye 
(the  retina  of  which  contains  only  rods,  and  which  is 
also  well  adapted  for  the  observation  of  the  electrical 
change  produced  by  light)  that  in  the  outer  part  of  the 
rods  of  quiescent  eyes  there  is  a  pigment  of  a  purple 
colour  derived  from  the  pigmented  layer  outside  of  Jacob's 
membrane,  and  on  exposure  of  the  eye  to  ordinary  light 
this  purple  changes  to  yellow  and  then  to  white.  On 
removal  of  the  light  the  pigment  slowly  reappears  in  the 
rods.  This  pigment  is  not  found  in  the  cones  of  the  retina 
of  other  animals,  and  hence  is  absent  in  the  yellow  spot. 
As  the  yellow  spot  is  the  seat  of  acute  vision  in  daylight  we 
must  infer  that  the  purple  pigment  is  not  essential  to  vision, 
but  we  must  not  conclude  from  this  that  it  has  no  visual 
function.  For  if  we  pass  from  darkness  to  bright  light, 
the  eye  at  first  is  dazzled  until  possibly  the  visual  purple 
is  bleached,  or  in  other  words,  until  the  eye's  sensibility 
to  light  is  diminished  ;  and,  on  the  other  hand,  if  the 
eye  has  been  exhausted  by  bright  light  we  do  not  see 
objects  well  in  a  dim  light  until  the  visual  purple  is  restored. 
In  a  dim  light,  the  pupil  of  the  eye  is  dilated,  and  rays 
affect  the  retina  round  the  yellow  spot.  It  would  thus 
appear  that  visual  purple  assists  vision  in  dim  light  while 
it  is  not  necessary  in  bright  light ;  but  as  we  have  a 
chemical  change  in  the  purple  pigment,  so  we  may  have 
in  the  yellow  spot  substances  which  undergo  chemical 
change,  although  this  be  not  manifest  to  the  observer. 
The  yellow  spot  is  thus  better  adapted  for  acuteness  of 
vision,  for  concentration  of  the  attention  upon  minute 
detail,  while  the  surrounding  parts  of  the  retina  are  more 
sensitive  to  the  action  of  light  and  more  fitted  for  observ- 


152  Physiology  of  the  Senses 

ing  bodies  emitting  or  reflecting  but  a  small  quantity  of 
light. 

Amount  of  Light  required  to  excite  the  Retina. — 

The  smallest  amount  of  light  that  will  excite  the  retina 
cannot  be  stated,  as  so  much  depends  upon  the  part 
of  the  eye  affected,  its  state  of  vigour  or  exhaustion, 
its  previous  education,  and  the  like.  Thus  the  sailor 
will  see  land  in  the  distance  which  is  imperceptible  to 
the  landsman  ;  the  Oriental  will  distinguish  shades  of 
colour  more  accurately  than  the  European ;  and  the  artist 
will  differentiate  where  the  untrained  eye  sees  but  one 
tint.  Again,  the  exhausted  eye  will  fail  to  see  what  is 
readily  perceptible  to  the  fresh  eye  of  one  newly  wakened 
from  sleep  ;  and  the  star,  whose  faint  light  is  unseen  by 
direct  vision,  may  be  seen  when  its  ray  meets  the  retina  a 
little  to  the  outside  of  the  yellow  spot.  Nay  more,  even 
when  we  are  enveloped  by  the  deepest  darkness,  and  when 
the  eyes  are  shut,  the  ordinary  field  of  vision  seems  still 
irradiated  by  a  faint  pervading  glow,  known  as  the  specific 
light  of  the  retina,  which  upon  slight  pressure  by  the  hands 
may  be  broken  up  into  a  mosaic  of  fleeting  patterns.  The 
sensations  thus  excited  by  pressure  are  called  phosge?tes. 
The  retinal  light  is  caused  by  changes  in  the  retina  due  to 
variations  in  the  blood  supply. 

Persistence  of  Retinal  Impressions. — The  substance  of 
the  retina  is  more  or  less  affected  according  to  the  brilliancy 
of  the  light  and  the  length  of  time  during  which  it  acts 
upon  the  eye.  A  feeble  light  acting  for  a  short  time  will 
leave  but  a  transient  effect,  while  a  strong  light,  such  as 
that  of  the  sun  or  of  the  electric  spark  acting  for  an  instant 
only,  may  give  rise  to  impressions  lasting  many  minutes, 
or,  if  the  exposure  be  prolonged,  even  to  permanent  damage 
to  the  eyesight.  If  we  look  directly  at  the  sun  and  then 
turn  our  eyes  to  the  ground,  or  towards  a  darkened  cloud, 


The  Sense  of  Sight 


i53 


the  image  of  the  sun  formed  upon  the  retina  has  been 
as  it  were  so  deeply  graven,  the  retinal  structure  has 
been  so  changed,  that  for  several  moments  we  fail  to 
see  the  object  towards  which  the  eyes  are  turned,  and 
we  see  a  round  red  spot,  or  several  red  spots,  if  the 
eyes  were  not  steady  when  the  sun  was  in  view.  This 
spot  is  a  spectrum  or  after-image  of  the  sun  projected 
outwards  upon  the  visual  field,  moving  with  every  move- 
ment of  the  eye,  and  seen  even  when  the  eyes  are  closed. 
If  a  piece  of  burning  wood  be  shaken  rapidly  to  and  fro,  we 
see  a  line  of  light,  because  adjacent  points  on  the  retina  are 
consecutively  stimulated,  and  the  fusion  of  the  after-images 
gives  the  sensation  of  continuity.  A  disk  with  alternate 
lines  or  sectors  of  black  and 
white  radiating  from  the 
centre  will,  when  rotated 
rapidly,  seem  to  have  a  uni- 
form gray  colour  due  to  the 
fusion  of  the  black  and  white 
spectra;  but  if  seen  by  the 
light  of  the  instantaneous 
electric  spark,  each  black  and  white  line  or  sector  will  be 
visible  because  the  time  of  illumination  and  consequent  stimu- 
lation of  the  retina  is  so  short  that  there  is  no  time  for  the 
superposition  of  the  images  one  upon  the  other.  Similarly, 
if  various  simple  colours  be  painted  on  the  disk,  their  spectra 
will,  on  rotation  of  the  disk,  be  fused  together,  giving  rise 
to  a  sensation  of  the  colour  due  to  their  combination. 
If  a  series  of  twenty  or  thirty  instantaneous  photographs 
be  taken  at  short  but  equal  intervals  of  time  of  an 
animal  performing  some  movement,  as,  for  example,  a 
horse  leaping  over  a  gate,  the  pictures  fixed  to  a  disk 
will,  when  rotating  quickly,  seem  to  coalesce  each  with 
its    predecessor    so    as    to    give    the    impression    of    the 


A  B 

Fig.  75. — The  disk  A  having  black  and 
white  sectors,  when  rotated  rapidly 
gives  an  even  gray  tint  as  in  B. 


154  Physiology  of  the  Senses 

horse  in  actual  movement.  This  is  the  principle  of  the 
toy  known  as  the  Thaicmatrope  or  Wheel  of  Life.  Since 
the  after-image  in  the  instances  above  mentioned  has  an 
appearance  similar  to  that  of  the  object  viewed,  it  is  called 
a  positive  after-image.  But  there  is  another  kind  of  after- 
image, the  7iegative,  which  is  due  to  a  slightly  different 
cause.  Suppose  we  look  fixedly  at  an  object  for  thirty  or 
forty  seconds,  so  that  the  eye  becomes  fatigued,  and  then 
turn  our  eyes  to  a  surface  of  uniform  tint,  we  will  see  an 
image  floating  on  the  wall  in  which  the  lights  will  be 
reversed — what  was  dark  will  be  light,  what  was  bright  will 
be  dim.  In  this  case  the  rays  of  light  reflected  from  the 
wall  have  most  effect  upon  those  parts  of  the  retina  which 
are  least  exhausted,  while  those  parts  formerly  much  stimu- 
lated will  now  look  dark,  not  being  so  easily  excited  to  action. 

The  persistence  of  retinal  impressions  is  probably  in 
part  the  cause  of  the  phenomenon  known  as  irradiation 
(see  p.  1 40).  The  eye  moving  rapidly  over  the  white 
surface,  and  being  more  affected  by  its  light,  the  dark 
area  seems  the  smaller.  It  may  also  be  that  there  is  a 
slight  dispersion  of  light  from  the  retinal  elements  directly 
affected  to  those  immediately  adjoining,  which  makes  the 
image  larger,  and  so  leads  to  an  erroneous  judgment  as  to 
the  size  of  the  white  object. 

A  further  and  most  interesting  illustration  of  the  per- 
sistence of  the  retinal  state  may  be  studied  as  follows  :  Look 
steadily  for  about  half  a  minute  at  a  disk  with  alternate 
black  and  white  sectors  which  is  being  slowly  rotated. 
Then  turn  the  eyes  to  a  sheet  of  paper  upon  which  a 
number  of  dark  spots  may  be  seen.  These  will  seem  to 
rotate  in  a  direction  contrary  to  that  in  which  the  disk  was 
turning.  The  effect  here  is  of  the  same  nature  as  the 
phenomenon  often  seen  on  the  deck  of  a  steamer.  If  we 
lean  over  the  side  of  the  vessel,  and  watch  the  water  as 


The  Sense  of  Sight  155 

the  vessel  glides  along,  it  soon  seems  as  if  the  ship  were 
stationary  and  the  water  near  us  in  rapid  motion  in  the 
direction  opposite  to  that  in  which  we  are  moving — the 
apparent  rapidity  gradually  diminishing  as  we  look  at  more 
remote  parts  of  the  water.  If  we  now  gaze  at  the  deck,  the 
part  near  us  will  seem  to  move  towards  the  bow  of  the  ship, 
the  rest  of  the  deck  remaining  fixed.  Different  parts  of  the 
retina  have  been  stimulated  by  rays  from  different  parts 
of  the  surface  of  the  water  apparently  moving  at  different 
rates.  But  when  the  whole  visual  field  is  occupied  by  the 
deck,  the  various  parts  of  which  are  fixed  relatively  to  each 
other,  the  persistence  of  the  retinal  impression  of  greater 
movement  in  one  part  of  the  visual  field  than  in  the  rest  of 
it  causes  us  to  imagine  that  parts  of  the  deck,  which  rela- 
tively to  the  rest  of  the  deck  are  stationary,  are  actually  in 
motion. 

5. — Sensation  of  Colour 

In  considering  the  physical  nature  of  light  (p.  1 1  5),  we  saw 
that  the  shade  of  colour,  according  to  the  most  likely  hypo- 
thesis, depends  on  the  rate  of  vibration  of  the  luminiferous 
ether,  and  that  solar  or  white  light  is  a  compound  of  all  the 
colours  in  definite  proportion.  A  body  which  reflects  solar 
light  to  the  eye  without  changing  this  proportion  appears  to 
be  white  ;  if  it  absorbs  all  the  light  so  as  to  reflect  no  light 
to  the  eye,  it  appears  to  be  black.  If  a  body  held  between 
the  eye  and  the  sun  transmits  light  unchanged  and  is 
transparent,  it  is  colourless  ;  but  if  translucent,  it  is  white. 
If  it  transmits  or  reflects  some  rays  and  absorbs  others,  it 
is  coloured.  If,  for  example,  it  absorbs  all  the  rays  of  the 
solar  spectrum  but  those  which  give  rise  to  the  sensation 
of  greenness,  we  say  that  the  body  is  green  in  colour. 
But  this  greenness  can  only  be  perceived  if  the  rays 
of  light  falling  on  the  body  contain  rays  having  the  special 


156 


Physiology  of  the  Senses 


vibratory  rate  that  is  required  for  this  special  colour.  For, 
if  we  use  as  our  light  any  other  pure  coloured  ray  of  the 
spectrum,  say  the  red,  its  rays  being  absorbed  the  body 
appears  to  us  to  be  black.  A  white  surface  seen 
in  a  red  light  seems  to  be  red,  in  a  green  light, 
green,  as  it  reflects  all  colours  alike,  absorbing  none. 
To  the  normal  eye  the  colour  depends,  then,  on  the 
nature  of  the  body  and  of  the  light  falling  upon  it,  and  the 
se?isatio7i  of  colour  only  arises  when  the  body  reflects  or 
transmits   the   special   rays   to    the   eye.       If  two   rays   of 


bed. 

Fig.  76. — Lambert's  method  for  studying  combinations  of  colour.  The  rays,  e.g., 
from  the  red  wafer  d  reflected  by  the  glass  plate  a  to  the  eye  E  are  pro- 
jected outwards  and  superposed  on  the  blue  wafer  b,  which  appears  of  a  rose 
colour. 

different  colour  affect  one  part  of  the  retina  at  the  same 
time,  they  are  fused  together,  and  we  have  the  sensation 
of  a  third  colour  different  from  its  cause.  Thus,  if  red 
be  removed  from  the  solar  spectrum,  all  the  others  com- 
bined will  give  a  sensation  of  a  greenish  yellow,  although 
we  cannot,  with  the  unaided  eye,  analyse  this  into  its  com- 
ponents. 

Fig.  76  shows  a  method  by  which  different -coloured 
rays  may  be  made  to  converge  from  two  bodies  on  the 
same  part  of  the  retina.     Von  Helmholtz  gives  the  follow- 


The  Sense  of  Sight 


i57 


ing  table  as  the  result  of  mixing  the  pure  colours  of  the 
spectrum  : — 


Y. 

B. 

G.             Y. 

R. 

R. 

Purple. 

t,               Dull 
Rose.      ^r  „ 

Yellow. 

Orange. 

Red. 

Y. 

Rose.     |  White. 

Yellow 
Green. 

Yellow. 

G. 

Pale 

Blue. 

Blue 
Green. 

Green. 

Bl. 

Indigo. 

Blue. 

V. 

V. 

Thus  a  mixture  of  red  and  violet  gives  purple,  of  yellow  and 
blue,  white.  Here  we  must  guard  against  a  possible 
error.  The  effect  of  say  yellow  and  blue  light  acting  at 
once  on  the  eye  is  to  cause  a  sensation  of  white  light ;  but 
if  we  mix  blue  and  yellow  pigments  the  mixture  looks 
green,  because  the  one  pigment  cuts  off  the  rays  at  the 
red  end,  the  other  those  at  the  violet  end  of  the  spectrum, 
and  the  only  rays  reflected  are  those  of  the  green  or  middle 
part  of  the  spectrum.  In  the  one  case  we  have  a  com- 
bination of  colours,  in  the  other  each  absorbs  a  part  of 
the  spectrum  previously  seen  when  the  pigments  were 
unmixed. 

Similarly,  if  the  colours  of  the  spectrum  be  painted  upon 


158  Physiology  of  the  Senses 

a  disk,  in  due  proportion  and  in  proper  series,  the  disk 
will,  when  quickly  rotated,  look  white.  This  is  due  to  a 
fusion  of  colour  effects,  not  to  a  mixture  of  the  pigments. 

Complementary  Colours.  —  When  one  colour  is 
separated  from  the  spectral  series,  the  rest,  as  we 
have  said,  may  be  combined  in  the  retina  to  give  a 
sensation  of  one  colour,  and  this  colour  will,  if  recom- 
bined  with  the  one  originally  separated,  give  the  sensa- 
tion of  white  light.  These  two  colours,  then,  are  said 
to  be  complementary  to  each  other,  and  every  colour  in 
the  spectrum  may  thus  be  said  to  be  the  complement  of 
all  the  others.  By  combining  colours  at  opposite  ends  of 
the  spectrum,  the  effect  of  the  intermediate  colours  may 
be  produced ;  but  the  lowest  and  highest  of  the  series,  the 
red  and  the  violet,  cannot  be  thus  formed.  They  may  be 
regarded,  therefore,  as  primary  colours  —  colours  which 
cannot  be  produced  by  the  fusion  of  others. 

If  to  red  and  violet  we  add  the  colour  whose  vibratory 
rate  is  about  midway  intermediate,  viz.  green,  we  may,  by 
their  combination,  give  rise  to  a  sensation  approaching 
that  of  white  light.  Consequently  these  three  colours  have 
been  designated  the  fundamental  colours. 

Colour  as  dependent  on  the  Retina.  —  Our  per- 
ception of  colour  depends,  however,  not  only  on  the 
physical  stimulus  of  light,  but  also  on  the  part  of  the 
retina  affected.  In  and  around  the  yellow  spot  where  the 
cones  are  most  numerous,  the  power  of  distinguishing 
shades  of  colour  is  greatest.  Instead  of  seven  colours  in 
the  spectrum  more  than  two  hundred  different  tints  may  be 
distinguished.  Outside  of  this  central  area  lies  a  middle 
zone  in  which  much  fewer  tints  are  seen,  these  being  con- 
fined, indeed,  to  shades  of  blue  and  yellow  ;  while  in  the 
front  part  of  the  retina  all  colour  tints  are  lost,  and 
objects  give  rise  simply  to  the  sensation  of  dark  shadowy 


The  Sense  of  Sight  159 

bodies  without  colour.       Moreover,   the  range  of  spectral 
colours  varies  with  the  individual. 

Colour  Blindness. — Every  colour  has  three  qualities  : 

( 1 )  hue,  or  tint,  as  when  we  speak  of  red,  green,  or  violet  ; 

(2)  purity,  or  degree  of  saturation  (due  to  a  greater  or  less 
admixture  with  white),  as  when  we  designate  a  red  or  green 
as  deep  or  pale  ;  and  (3)  brightness,  or  intensity,  or  lumin- 
osity, as  when  we  describe  the  tint  of  a  red  rose  as  dark 
or  bright.      On  comparing  two  colours  we  say  they  are 
identical    when    they    agree    as    to    these    three    qualities. 
Observation  has  shown  that  in  thus  assorting  colours,  about 
ninety-six  out  of  every  hundred  men  will  agree  as  to  identity 
or  difference  of  colour,  and  may  be  said  to  have  7iormal 
colour  vision,   while  the  remaining  four  men  will  show  a 
defective  perception  of  colour,  and  are  called  colour  blind. 
It  is  curious  that  colour  blindness  is  about  ten  times  less 
frequent  in  the  female  sex.     This  condition  is  congenital 
and  incurable.     It  is  due  to  some  unknown  peculiarity  of 
the  retina,  or  nerve  centres,  or  both,  and  it  is  to  be  dis- 
tinguished   from    transient     colour    blindness,    sometimes 
caused  by  the  excessive  use   of  tobacco  and   by  disease. 
There  is  probably  no   such  condition   as   absolute   colour 
blindness,  in  the  sense  of  total  insensibility  to  colour  ;  a  few 
rare  cases  have  been  noticed  in  which  there  was  apparently 
only  one  colour  sensation  ;  a  few  cases  occur  of  failure  to 
distinguish  blue  from  green,   and  insensibility  to  violet  is 
rare.     The    common    form    of   defective    colour   vision    is 
Daltonism  or  red-green  blindness,  of  which  there  are  two 
varieties — the   red -blind  and    the   green -bliiid.      In    each 
variety  there  are  many  gradations  of  sensibility.      To  the 
red-blind  red  appears  as  a  dark  green  or  greenish  yellow, 
yellow  and  orange  appear  as   dirty  green,  while  green  is 
green  and  brighter  than  the  green  of  the  yellow  and  orange. 
A  green-blind  person,   on  the  other  hand,   would  call  red 


160  Physiology  of  the  Senses 

dark  yellow,  yellow  would  be  yellow  except  a  little  lighter 
than  the  red  he  calls  dark  yellow,  and  green  would  be 
described  as  pale  yellow.  When  asked  to  look  through  a 
spectroscope  at  the  spectrum,  the  extreme  or  low  red  is 
absent  to  the  red  blind,  and  the  brightest  part  of  the 
spectrum  appears  to  him  to  be  the  green,  while  to  the 
normal  eye  and  to  the  green-blind  eye  the  spectrum  is 
most  luminous  in  the  yellow. 

Seeing  that  green  lights  imply  safety,  'and  red  lights 
danger,  on  our  railways,  and  that  in  navigation  a  green  or 
red  light  on  the  port  or  starboard  side  shows  the  course  a 
vessel  is  taking,  it  is  evident  that  no  one  who  is  red-  or 
green-blind  should  be  employed  in  the  services,  and 
accordingly  various  tests  are  now  in  use  for  the  detection  of 
such  defects.  The  most  efficient  is  the  wool-test  of  Holm- 
gren, which  consists  of  three  skeins  of  wool  dyed  with 
standard  test  colours,  namely,  a  light  green,  a  pale  purple  or 
pink,  and  a  bright  red.  Other  skeins  of  reds,  oranges, 
yellows,  yellowish  greens,  pure  greens,  blue  greens,  violets, 
purples,  pinks,  browns,  and  grays,  all  called  confusion  colours, 
are  provided,  and  the  examinee  is  requested  to  select  one 
and  match  it  with  one  of  the  test  colours.  Suppose  the 
light  green  skein  is  shown  first.  If  the  examinee  matches 
grays,  brownish  grays,  yellows,  orange,'  or  faint  pink  with 
this,  he  is  colour  blind.  Then  he  is  shown  the  purple 
skein.  If  he  matches  with  this  blue  or  violet  he  is  red- 
blind,  but  if  he  selects  only  gray  or  green  he  is  green-blind. 
Finally,  he  may  be  shown  the  red  skein,  having  a  bright  red 
colour,  like  the  red  flag  used  on  railways.  A  red -blind 
person  will  then  match  with  this  green  or  shades  of  brown, 
which  to  a  normal  eye  seem  darker  than  red  ;  while  if  he 
be  green-blind  he  will  select  shades  of  these  colours  which 
look  lighter  than  red.  Violet  blindness  is  recognised  by 
the  examinee  confusing  red  and  orange  with  purple. 


The  Sense  of  Sight  161 

Coloured  after-images. — The  power  of  the  retina  in 
distinguishing  colours  depends  also  upon  its  freedom  from 
fatigue.  As  there  may  be  after-images  of  form,  so  there 
may  be  after-images  of  colour,  and  these  after-images  may  be 
negative  or  positive.  \i  positive,  we  see  with  the  eyes  shut  the 
same  colour  as  we  have  just  been  looking  at ;  if  negative,  we 
see  the  complementary  colour,  and  as  we  continue  examining 
it  we  find  the  colour  changing  and  fading  away,  the  lighter 
tints  merging  into  the  darker.  The  eye  fatigued  by  gazing 
at  a  red  square,  will,  when  turned  to  a  white  surface,  seem 
to  see  a  bluish-green  square  on  the  white  ground,  for  the 
fatigued  eye  responds  more  readily  to  the  stimulus  of  the 
other  colours  of  the  spectrum  ;  and  these  give,  when  fused, 
the  complementary  colour  (p.  158).  Similarly,  a  white 
square  seen  against  a  bluish-green  background  will  have 
a  reddish  tint,  probably  because  the  eye  moving  quickly 
over  the  coloured  field,  and  becoming  thereby  fatigued, 
responds  more  readily  to  the  red  rays  in  the  white  light 
than  to  its  other  component  parts.  This  is  known  as  the 
phenomenon  of  contrast. 

Theories  of  Colour  Vision.  —  How  comes  it  that  we 
can  perceive  differences  in  colour  ?  This  question  has 
never  been  satisfactorily  answered,  because  the  changes 
caused  in  the  retina  by  the  action  of  light  are  too  minute 
to  allow  of  direct  observation.  Many  hypotheses  have 
been  framed,  but  none  of  them  meets  all  the  requirements 
of  the  case.  We  may  look  for  the  cause  in  various  direc- 
tions. We  migh't  suppose  a  molecular  vibration  to  be  set 
up  in  the  nerve-endings  synchronous  with  the  undulations 
of  the  luminiferous  ether,  without  any  change  in  the 
chemical  constitution  of  the  sensory  surface  ;  and  we  might 
suppose  that  where  various  series  of  waves  corresponding 
to  different  colours  act  together,  these  are  fused  together, 
or  interfere  with  each  other  in  such  a  way  as  to  give  a 

M 


162 


Physiology  of  the  Senses 


vibration  of  modified  form  or  rate  corresponding  somehow  to 
the  sensation  arising  in  consciousness.  Or  again,  we  might 
suppose  that  the  effect  of  different  -  coloured  rays  is  to 
promote  or  retard  chemical  changes  in  the  sensory  surface, 
which  again  so  affect  the  sensory  nerves  as  to  give  rise  to 
differing  states  in  the  nerves  and  nerve  centres  with  differing 
concomitant  sensations.  The  former  of  these  lines  of 
thought  guided  Thomas  Young,  the  great  expounder  of  the 


H  G 

Violet.      Indigo.  Blue.  Green.  Orange  yellow. 

Fig.  77. — Diagram  to  illustrate  the  Young-Helmholtz  theory  of  colour  vision. 
The  lines  with  the  letters  B,  C,  D,  etc.,  helow  the  curves  indicate  certain 
fixed  lines  in  the  solar  spectrum,  whose  wave-length  has  been  determined. 
Take  D,  the  height  of  the  two  curves  above  it  indicates  the  degrees  of  stimu- 
lation of  the  two  sensations  red  and  green  that  produce  orange-yellow. 
Again,  at  E  we  see  a  mixture  of  the  three  sensations  that  produce  spectral 
green.     (Report  of  the  Committee  of  the  Royal  Society  on  Colour  Vision.) 

undulatory  theory  of  light,  in  his  attempt  at  explaining 
colour  perception  ;  and  his  theory  adopted  and  worked  out 
by  von  Helmholtz  has  been  received  with  much  favour. 
He  supposed  that  there  are  three  fundamental  colour  sensa- 
tions— red,  green,  and  violet — by  the  combination  of  which 
all  other  colours  may  be  formed,  and  that  there  are  in  the 
retina  three  kinds  of  nerve  elements,  each  of  which  is 
specially  responsive  to  the  stimulus  of  one  colour,  and  much 


The  Sense  of  Sight  163 

less  so  to  the  others.  If  a  pure  red  colour  alone  act  on  the 
retina,  only  the  corresponding  nerve  element  for  red  sensa- 
tion would  be  excited,  and  so  with  green  and  violet.  But 
suppose  the  colour  be  mixed,  then  the  nerve  elements  will 
be  set  in  action  in  proportion  to  the  amount  of  constituent 
excitant  rays  in  the  colour.  Thus,  if  all  the  nerve  elements 
be  set  in  action,  we  shall  have  white  light ;  if  that  corre- 
sponding to  the  red  and  green,  the  resultant  sensation  will 
be  orange  or  yellow ;  if  mainly  the  green  and  violet,  the 
sensation  will  be  blue  or  indigo,  and  the  like.  Von  Helm- 
holtz  succinctly  puts  it  as  follows  : — 

(1)  Red  excites  strongly  the  fibres  sensitive  to  red,  and  feebly 

the  other  two — sensation,  red. 

(2)  Yellow  excites  moderately  the  fibres  sensitive  to  red  and 

green,  feebly  the  violet — sensation,  yellow. 

(3)  Green  excites  strongly  the  green,  feebly  the  other  two — 

sensation,  green. 

(4)  Blue  excites  moderately  the  fibres  sensitive  to  green  and 

violet,  and  feebly  the  red — sensation,  blue. 

(5)  Violet  excites  strongly  the  fibres  sensitive  to   violet,   and 

feebly  the  other  two — sensation,  violet. 

(6)  When  the  excitation  is  nearly  equal  for  the  three  kinds  of 

fibres,  then  the  sensation  is  white. 

Another  mode  of  expressing  the  theory  is  to  say  that 
each  primary  sensation  of  red,  green,  and  violet  is  excited 
in  some  degree  by  almost  every  ray  of  the  spectrum,  but 
the  maxima  of  excitation  occur  at  different  places,  while 
the  strength  of  stimulation  in  each  case  diminishes  in  both 
directions  from  the  maximum  point.  Thus,  when  the  three 
sensations  are  equally  excited,  white  light  is  the  result ; 
green  is  caused  by  a  very  weak  violet  sensation,  a  stronger 
red,  and  a  still  stronger  green  sensation.  At  each  end  of 
the  spectrum  we  have  only  the  simple  sensations  of  red 
and  violet,  and  all  the  intermediate  colour  sensations  are 
compounds  of  varying  proportions  of  the  three  primaries. 


164  Physiology  of  the  Senses 

According  to  this  theory,  red  blindness  is  attributable  to 
the  absence  of  the  red  sensation,  and  green  blindness  to  the 
absence  of  the  green  sensation.  When  the  green  and 
violet  sensations  are  equal  in  amount,  a  red -blind  person 
sees  what  is  to  him  white,  and  when  the  red  and  violet  are 
equal  a  green-blind  person  will  have  a  sensation  of  what  in 
turn  is  to  him  white,  although  to  the  normal  eye  these 
parts  are  bluish  green  in  the  one  case  and  green  in  the 
other,  as  the  green  sensation  is  in  each  added  to  the 
sensations  of  red  and  blue. 

But  while  this  theory  explains  certain  phenomena  of  colour 
blindness,  of  after-images,  and  of  colour  contrast,  it  is  yet 
open  to  serious  objections.  There  is  no  proof,  one  way  or 
other,  of  the  existence  of  three  kinds  of  nerve  elements 
corresponding  to  the  three  fundamental  colour  sensations. 
Again,  it  does  not  explain  how  red  should  have  to  the 
colour-blind  person  a  similar  appearance  to  green,  or  how 
it  should  give  rise  to  a  sensation  of  colour  at  all,  any  more 
than  heat  rays  which  are  invisible.  Further,  if  red  rays 
are  a  necessary  constituent  of  white  light,  the  colour  blind 
should  not  be  able  to  see  white  as  we  do,  nor  to  distinguish 
white  from  bluish  green — the  complementary  colour  of  red. 
And  yet  such  distinctions  can  be  made,  although  it  may  be 
argued  that  a  colour-blind  person  does  not  see  white  in  the 
same  sense  as  white  is  white  to  a  person  having  normal 
colour  vision.  A  strong  objection  to  the  Young-Helmholtz 
theory  is  that  in  cases  of  colour  blindness  following  injury 
to  the  eye,  only  the  blue  of  the  spectrum  is  seen,  all  the 
rest  appearing  as  white.  Here  it  is  impossible  to  under- 
stand how  a  sensation  of  white  can  be  experienced  if  the 
sensations  of  red  and  green  are  lost,  for  the  theory  is  that 
white  can  only  be  experienced  when  the  sensations  of  red, 
green,  and  violet  are  all  three  present. 

Stanley  Hall  likewise  adopts  an  anatomical  basis  for  his 


The  Sense  of  Sight  165 

theory  of  colour  perception.  He  holds  that  only  the  cones 
are  sensitive  to  colours,  and  that  these  may  be  regarded  as 
built  up  of  a  series  of  disks  like  a  pile  of  coins,  the  lowest 
of  which  is  the  largest.  Different  disks  respond  to  different 
colour  tones,  and  give  rise  to  different  excitations  of  the 
nerve  centres.  While  the  disk  formation  of  the  cones  is 
undoubted,  this  theory  is  open  to  the  same  objections,  on 
subjective  grounds,  as  that  of  Young  and  von  Helmholtz. 

Other  theories  of  colour  perception  proceed  upon  the 
assumption  of  chemical  changes  in  the  retina  under  the 
influence  of  light.  That  light  does  play  an  important  part 
in  physiological  action  is  a  well-known  fact.  Green  plants, 
for  instance,  can  only  grow  healthily  when  exposed  to  the 
light ;  if  kept  in  a  dark  chamber  they  quickly  blanch,  and  use 
up  only  the  reserve  material  stored  up  in  themselves,  because 
they  have  no  longer  the  power  of  obtaining  carbon  from  the 
carbonic  acid  of  the  air.  And  yet,  though  this  is  so,  it  is  also 
known  that  direct  rays  of  light  have  a  retarding  influence  on 
the  growth  of  certain  parts  of  plants.  If  a  plant  is  placed  in 
a  window,  it  bends  outwards  towards  the  light,  because  the 
side  of  the  stem  away  from  the  light  grows  the  faster ; 
similarly  leaves  of  plants  grown  in  the  dark,  like  rhubarb, 
have  long  thin  stalks  which  have  derived  their  nourishment 
from  the  root,  and  have  not  been  affected  by  light.  So 
Hering  holds  with  regard  to  the  retina. 

According  to  Hering' s  theory  certain  fundamental  sensa- 
tions are  excited  by  light  or  by  the  absence  of  light.  These 
are  white,  black,  red,  yellow,  green,  and  blue,  and  they  may 
be  arranged  in  three  pairs,  the  one  colour  in  each  pair  being 
complementary  to  the  other,  thus — white  to  black,  red  to 
green,  and  yellow  to  blue.  Hering  further  supposes  that 
when  rays  of  a  certain  wave-length  fall  on  visual  substances 
existing  in  the  retina  destructive  changes  occur,  while  rays 
having    other    wave  -  lengths    cause    constructive    changes. 


i66 


Physiology  of  the  Senses 


Thus,  suppose  a  red-green  visual  substance  exists  of  such  a 
nature  that  when  destructive  and  constructive  changes  occur 
no  sensation  is  experienced,  then  when  destructive  changes 
are  in  excess  by  the  action  of  light  of  a  certain  wave-length 
there  is  a  sensation  of  red,  and  when  constructive  changes 
occur  by  the  action  of  shorter  waves  the  sensation  is  green. 
In  like  manner  a  yellow-blue  visual  substance  by  destruc- 
tive changes  gives  a  sensation  of  yellow,  and  by  construc- 

Yb  W  gr 


»  B  G  Y  O  R 

Fig.  78. — Diagram  to  illustrate  Hering's  theory  of  colour  vision.  The  vertical 
shading  represents  the  red  and  green,  and  the  horizontal  shading  the  yellow 
and  blue,  antagonistic  pairs  of  sensations.  The  thick  line  indicates  the  curve 
of  the  white  sensation.  All  above  the  line  X  X  indicates  destructive  changes 
in  the  retinal  substances,  and  all  below  constructive  changes.  See  text. 
(Report  of  the  Committee  of  the  Royal  Society  on  Colour  Vision.) 

tive  changes  a  sensation  of  blue  ;  and  a  white-black  visual 
substance  by  destructive  changes  gives  white,  and  by 
constructive  changes  black.  The  member  of  each  pair  is 
thus  antagonistic  as  well  as  complementary.  The  red-green 
and  yellow-blue  substances  are  tuned,  as  it  were,  to  rays  of 
different  wave-length.  Thus,  in  the  red  end  of  the  spectrum, 
the  rays  cause  great  destruction  of  the  red-green  substance, 
while  they  have  no  effect  on  the  yellow-blue  substance. 
Hence   the   sensation   is   red.      Again,   the   shorter  waves 


The  Sense  of  Sight  167 

which  correspond  to  the  yellow  of  the  spectrum  cause  great 
destruction  of  the  yellow -blue  substance,  while  their  de- 
structive and  constructive  effects  on  the  red-green  substance 
neutralise  each  other.  Hence  the  sensation  is  yellow. 
Still  shorter  waves,  corresponding  to  green,  cause  construc- 
tion of  the  red-green  substance,  while  their  influence  on  the 
yellow-blue  substance  is  neutral,  and  hence  the  sensation  is 
green.  Again,  the  shorter  blue  waves  cause  construction 
of  the  yellow-blue  substance,  while  their  action  on  the  red- 
green  substance  is  neutral,  and  hence  the  sensation  is  blue. 
At  the  blue  end  the  short  waves  are  supposed  to  cause 
destruction  of  the  red-green  substance,  and  thus  give  violet 
by  adding  red  to  blue.  Orange  is  caused  by  excess  of 
destructive  changes,  and  greenish-blue  by  excess  of  con- 
structive changes  in  both  substances.  Finally,  when  all 
the  rays  of  the  spectrum  fall  on  the  retina,  the  constructive 
and  destructive  changes  in  the  red-green  and  yellow-blue 
substances  neutralise  each  other,  but  the  destructive 
changes  are  great  in  the  white-black  substance,  and  we  call 
the  effect  white.  Colour  blindness,  in  the  form  of  red-green 
blindness,  is,  according  to  this  theory,  due  to  the  absence 
of  the  red-green  substance,  the  other  two  substances 
remaining.  The  phenomena  of  coloured  after-images  are 
thus  accounted  for  : — 

Suppose  the  retina  to  be  acted  on  by  red  light,  destruc- 
tion of  material  takes  place — the  effect  continuing,  it  may 
be  for  a  time,  after  withdrawal  of  the  red  light — giving 
the  positive  after-image.  Then  comes  the  upbuilding  of 
the  material  under  the  influence  of  nutrition,  assisted  by 
the  action  of  light  of  shorter  wave-lengths,  and  the  negative 
after-image — green — is  perceived.  So  with  yellow  and  blue, 
and  white  and  black.  That  such  differences  of  chemical 
action  are  possible,  or  probable,  we  may  well  believe  from  a 
consideration  of  the  variation  in  the  actinic  effect  of  different 


1 68  Physiology  of  the  Senses 

rays  of  the  spectrum,  and  from  the  action  of  light  upon  the 
pigments  of  the  retina.  This  theory  is  also  in  harmony 
with  what  has  been  observed  in  connection  with  many  other 
processes  in  the  body,  such  as  secretion,  innervation,  and 
the  like,  in  which  tissues,  having  reached  their  highest  point 
of  vitality  through  nutrition,  disintegrate  during  functional 
activity.  Serious  objections  to  the  theory  have,  however, 
been  raised.  One  is  thus  stated  by  Ladd 1 : — "  A  light 
composed  of  red  and  green  may  be  made  to  seem  to  the  eye 
the  same  as  a  light  composed  of  yellow  and  blue.  If,  then, 
the  eye  is  fatigued  to  red,  instead  of  the  red-green  mixture 
appearing  greenish,  and  so  distinguishable  from  the  yellow- 
blue  mixture,  they  both  appear  the  same  to  the  fatigued  eye." 
It  has  also  been  pointed  out  that  the  two  sensations  of  each 
pair  do  not  always  coexist.  One  may  be  present  and  the 
other  absent.  Thus,  when  the  intensity  of  the  light  of  the 
spectrum  has  been  much  reduced,  the  green  persists  long 
after  the  red  has  disappeared  ;  and  after  the  excessive  use 
of  tobacco,  yellow  may  disappear,  and  blue  is  the  only 
sensation  left.  One  is  also  at  a  loss  to  understand  how 
colour  sensations,  so  different  from  one  another  as  red  and 
yellow,  can  be  alike  due  to  destructive  changes  of  retinal 
substances,  or  how  yellow  and  green,  whose  periods  of 
vibration  are  so  nearly  alike,  can  give  such  antagonistic 
physiological  effects.  Such  considerations  demand  the 
existence  not  of  one  but  of  three  visual  substances.  On 
the  whole,  however,  speculative  as  it  is,  Hering's  theory 
accounts  for  a  larger  number  of  the  phenomena  of  colour 
vision  than  that  of  Young  and  von  Helmholtz. 

In  a  new  edition  of  his  great  work,  Handbitch  der  Physio- 

logiscJwi   Optik,   now   appearing   in   parts,  von    Helmholtz 

reviews  the  subject  of  colour  vision,  and  materially  modifies 

the  theory  as  first  announced  by  him  about  1856,  and  since 

1  Ladd,  Outlines  of  Physiological  Psychology,  p.  268. 


The  Sense  of  Sight  169 

then  termed  the  Young-Helmholtz  theory.  He  now  states 
that  luminosity  or  brightness  plays  a  more  important  part  in 
our  perceptions  of  colour  than  has  been  supposed.  He  also,  by 
analysing  the  colours  of  the  spectrum  with  great  care,has  been 
able,  from  these  data,  to  determine  three  fundamental  colour 
sensations,  the  first  red  {a),  which  is  a  highly  saturated 
carmine-red,  the  second  greeti  (b)  like  the  green  of  vegeta- 
tion, and  the  third  bhie  (c)  like  ultramarine.  Each  spectral 
colour  is  made  up  of  certain  proportions  of  these  funda- 
mental colours,  or  a  combination  of  two  of  them  added  to 
a  certain  amount  of  white.  Thus  100  parts  of  green  are 
composed  of  15  of  a,  51  of  b,  and  34  of  c  \  or,  to  take 
other  examples,  spectral  red  will  contain,  per  cent.,  42  of  «, 
1  of  b,  and  5  7  of  white  ;  yellow,  11  of  a,  1 4  of  b,  and  7  5  of 
white;  and  blue,  2  of  a,  1 1  of  c,  and  87  of  white.  The 
white  gives  the  element  of  brightness.  According  to  this 
view,  it  is  not  necessary  to  suppose  that  in  the  red-blind 
the  red-perceiving  elements  are  awanting,  or  that  in  the 
green-blind  the  green-perceiving  elements  are  absent,  but 
that  these  elements  may  be  stimulated  with  intensities 
different  from  those  affecting  the  normal  eye.  Suppose 
that  in  the  eye  of  a  colour-blind  person  the  curves  of  inten- 
sity representing  the  red  and  green  coincided,  or,  in  other 
words,  that  the  elements  responsive  to  red  and  green  in  the 
abnormal  eye  were  stimulated  with  intensities  equal  to  that  of 
red  in  a  normal  eye,  the  sensation  would  be  yellow,  as  we 
find  to  be  the  case  in  so-called  green  blindness.  Again,  if 
in  a  similar  way  the  red  curve  coincided  as  regards  inten- 
sity with  the  green,  the  general  effect  would  be  that  of  a 
red-blind  person,  the  red  end  of  the  spectrum  would  appear 
to  be  green,  and  no  red  would  be  visible.  This  theory  does 
not  profess  to  state  what  may  occur  in  the  retina  in  the 
way  of  chemical  change,  as  is  attempted  in  the  theory  of 
Hering". 


170  Physiology  of  the  Senses 

Captain  Abney  and  Major-General  Festing  have  also 
investigated  the  question  of  colour  sensation  by  photo- 
metrical  methods,  and  have  been  able  to  mark  out  the 
curves  of  luminosity  both  of  normal  and  of  colour-blind 
eyes.  Their  observations  support  the  Young-Helmholtz 
theory,  and  indicate  clearly  that  the  peculiar  sensations  of 
colour  experienced  by  colour-blind  people  are  due  either 
to  the  different  intensities  of  the  three  primary  colour  sensa- 
tions, or  to  the  absence  of  one  or  more  of  those  sensa- 
tions.1 

6. — Binocular  Vision 

Having  considered  the  eye  as  an  optical  instrument, 
we  have  next  to  inquire  how  the  two  eyes  act  together, 
and  what  are  the  advantages  of  binocular  over  monocular 
vision. 

Movements  of  the  Eye. — When  we  wish  to  change  our 
field  of  view,  we  may  do  so  either  by  moving  the  head  as  a 
whole,  or  the  eyes  alone.  The  eyes  move  very  freely  in 
their  sockets,  but,  as  we  shall  see,  their  movements  have 
certain  limitations.  The  orbits — the  cavities  of  the  skull  in 
which  the  eyes  are  set — contain  the  muscles  by  which  the 
eyes  are  moved,  nerves,  vessels,  glands,  connective  tissue, 
and,  lastly,  a  considerable  quantity  of  fat,  which  forms  an 
elastic  cushion  on  which  the  eyeballs  rest.  The  depth  of 
setting  of  the  eyes  in  the  orbits  varies  in  different  people, 
and  in  the  same  person  from  time  to  time  ;  but,  as  a  general 
rule,  the  eyes  are  so  situated  that  one  may,  without  moving 
the  head,  look  outwards  and  slightly  backwards  to  either 
side.  We  may  readily  prove  this  by  standing  erect  with 
the  back  of  the  head  against  a  wall.  If  some  bright  object 
on  a  level  with  the  eyes,  and  touching  the  wall,  be  moved 
gradually  outwards  from  the  head,  it  will,  at  a  certain  point 
1  Philosophical  Transactions,  1886,  1888,  1892. 


The  Sense  of  Sight  171 

(about  8  inches  to  1  foot),  become  visible.  The  head  being 
kept  fixed,  a  similar  point  may  be  determined  for  the  other 
side  of  the  head  ;  and  a  straight  line  drawn  from  these 
points  through  the  outer  angles  of  the  orbits  will  be  found 
to  meet  at  an  angle  of  about  900  ;  or,  in  other  words,  if  the 
head  be  considered  as  placed  within  a  circle,  only  one 
quadrant  of  the  circle  is  shut  off  from  the  visual  field, 
namely,  that  in  which  the  head  lies. 

The  movements  of  protrusion  and  retraction  of  the  eye- 
balls are  involuntary,  and  of  little  importance  for  vision,  but 
rotatory  movements  of  the  eyeball  require  careful  considera- 
tion.  These  take  place  round 


a  centre  of  rotation  which, 
according  to  Donders,  lies 
1-77  mm.  behind  the  centre  of 
the  visual  axis,  or  16-05  mm- 
from  the  vertex  of  the  cornea. 
We  may  conceive  of  three 
axes  passing  through  this 
centre,  an  antero-posterior,  a 

Fig.  79. — Diagram  to  illustrate  the  fact 

transverse,    and    a    vertical      that  we  can  see  objects  in  a  plane 

axis,  and  each  Of  these    axes         behind    a    transverse    vertical    plane 

.  through  the  two  eyes. 

may  be  regarded  as  lying  in 

planes  which,  passing  through  the  coats  of  the  eyeball,  divide 
the  ball  into  two  nearly  equal  parts,  an  upper  and  lower,  an 
outer  and  inner,  and  an  anterior  and  posterior.  These  axes 
and  planes  have  a  certain  fixed  position,  the  primary  position, 
with  reference  to  the  orbit  when  the  eye  is  at  rest.  If  the 
eyeballs  rotate  on  the  antero- posterior  or  visual  axes  from 
the  primary  position,  either  vertically  or  horizontally, 
the  eyes  are  said  to  have  assumed  a  secondary  position, 
and  a  tertiary  position  if  they  move  in  an  oblique  plane, 
so  as  to  look  inwards,  and  at  the  same  time  upwards 
or    downwards.       In    the    secondary    position,    there    can 


172 


Physiology  of  the  Senses 


be  no  rotation  of  the  eye  around  the  antero- posterior 
axis,  but  in  the  tertiary  position  there  is  always  more 
or  less  rotation  upon  all  three  fundamental  axes — on 
the  antero-posterior,  for  example,  it  may  be  even  more 
than  io°.  Such  circular  rotation,  or  rolling  of  the  eyes, 
takes  place  when  the  head  leans  towards  either  shoulder. 
In  this  case  the  direction  of  rolling  is  such  as  tends  to 
counteract  the  deviation  of  the  head. 

The  Ocular  Muscles, — The  movements  of  the  eye  are 
caused  by  the  action  of  six  muscles.      Four  of  these,  the 

,  direct  muscles  or  recti  (Fig. 
80),  pass  forwards  from 
the  back  part  of  the  orbit 
to  be  inserted  severally  on 
the  upper,  lower,  inner, 
and  outer  sides  of  the  eye- 
ball, and  their  action  is 
easily  understood.     When 

Fig.  80.— Diagram  of  muscles  of  right  eye.     the  itmer  muscle  Contracts, 
1,    Elevator   of  the   eyelid ;   2,    superior     ,1  ,    , 

',.  ,  .    .        .     '.   '         ,      the  eye  rotates  on  its  ver- 

obhque  muscle  ;  3,  superior  direct  muscle ;  J 

4,  4',  external  direct  muscle  cut  in  order    tical     axis     and     looks     in- 
to   show   part   of  the   optic   nerve,    and         „j„       „j       i_         .1  / 

4,    .  ,      .  ,.     .         ,        . ',  .      wards,  and  when  the  outer 

7,  the  internal  direct  muscle  ;  5,  interior  ' 

oblique  muscle  ;  6,  inferior  direct  muscle,     acts,   OUtwards.     When  the 

(Schwaibe.)  upper  contractS)  the  eyeball 

rotates  upon  its  transverse  or  horizontal  axis  and  the  eye 
looks  upwards  ;  when  the  lower  contracts,  the  eye  looks 
down. 

It  must  be  borne  in  mind,  however,  that  as  the  upper 
and  lower  recti  pass  somewhat  obliquely  outwards  to  their 
places  of  insertion  in  the  eyeball,  there  is  a  slight  inward 
direction  given  by  them  to  the  line  of  vision  in  addition  to 
the  deviation  up  or  down.  To  correct  the  inward  devia- 
tion, and,  in  general,  to  give  circular  rotation  to  the  eye, 
two  oblique  muscles  exist.     The  upper  (superior  oblique), 


The  Sense  of  Sight  173 

passing  forwards  along  the  inner  wall  of  the  orbit,  passes 
through  a  small  fibrous  ring  attached  to  the  bone,  and  turns 
like  a   rope   on   a   pulley  backwards   and  outwards  to  be 
inserted  into  the  upper  surface  of  the  eyeball.      The  other 
(inferior  oblique),  arising  from  the  front  part  of  the  inner 
wall  of  the  orbit,  passes  backwards  and  outwards  under  the 
eyeball,   and  is   inserted   into   its  outer  part.      The    upper 
oblique  muscle  rotates  the  eye  downwards  and  outwards,  the 
lower  upwards  and  outwards.     The  outer  or  inner  direct 
muscle  (external  or  internal  rectus)  alone  suffices  to  rotate 
the  eyes  outwards  or  inwards  in  a  horizontal  plane.     To 
cause  upward  or  downward  rotation  vertically,  the  upper 
rectus  and  the  lower  oblique,  or  the  lower  rectus  and  upper 
oblique,  come  into  play.     For  oblique  movements,  the  two 
recti  adjoining  the  quadrant,  into  which  the  fore  part  of  the 
visual  axis  moves,  together  with  one  of  the  oblique  muscles, 
act  simultaneously.      Further,  since  we  habitually  use  both 
eyes  in  looking  at  an  object,  it  will  be  readily  understood 
how  delicate  and  accurate  the  co-ordination  of  the  muscular 
action  must  be.      In  looking  upwards  or  downwards  similar 
sets  of  muscles  will  of  course  come  into  play  ;  but  in  look- 
ing sideways  the  outer  set  of  one  orbit  acts  at  the  same 
time  as  the  inner  of  the  other,  and,  in  converging  the  eyes 
upon  a  near  object,  the  two  inner  sets  will  co-operate.     The 
ocular  muscles  in  all  voluntary  movements  tend  to  render 
the  view  of  the  object  we  wish  to  look  at  distinct,  by  the 
formation  of  its  image  on  the  yellow  spot,  and  they  cannot 
act  so  as  to  lead  to  the  formation  of  images  on  non-corre- 
sponding points  of  the  retina  (see  p.  177).    We  cannot  look 
upwards  with  one  eye  while  the  other  eye  is  turned  down- 
wards, nor  can  we  look  with  the  right  eye  to  the  right  and 
the  left  eye  to  the  left  at  the  same  moment.      It  has  been 
pointed  out  by  Le  Conte  that  in  drowsiness,  intoxication, 
and  death,  when  the  eyes  are  in  a  purely  passive  state,  the 


174 


Physiology  of  the  Senses 


visual  axes  diverge  slightly,  and  for  this  reason  the  intoxi- 
cated man  sees  double.  Le  Conte  attributes  this  to  the 
divergence  of  the  axes  of  the  orbits  of  the  human  skull,  and 
holds  it  probable  that  "in  a  state  of  perfect  relaxation  or 
paralysis  of  the  ocular  muscles  the  optic  axes  coincide  with 
the  axes  of  the  conical  eye-sockets,  and  that  it  requires 


Fig.  Si. — Vertical  section  through  the  left  orbit  and  its  contents  in  the  orbital 
axis  and  with  eyelids  open,  a,  Frontal  bone  above  orbit ;  b,  upper  jaw-bone 
below  orbit ;  c,  thickened  bone  for  eyebrow  ;  d  upper,  d'  under  eyelid  with 
eyelashes  ;  e,  e,  meeting  of  conjunctiva?  of  eyelid  and  eyeball ;  /,  muscle  that 
elevates  upper  eyelid  ;  g,  superior  direct  muscle  ;  g\  inferior  direct  muscle  ; 
h,  cross  section  through  inferior  oblique  muscle  ;  i,  optic  nerve  ;  2,  cornea  ; 
3,  anterior  chamber;  4,  lens;  5,  vitreous  humour.     (Schwalbe.) 

some  degree  of  muscular  contraction  to  bring  the  optic 
axes  to  a  state  of  parallelism,  and  still  more  to  one  of  con- 
vergence, as  in  every  voluntary  act  of  sight." *  The 
doubling  of  the  image  caused  by  external  deviation  of  the 
fore  part  of  the  visual  axes  may  be  studied  if  we  press  upon 
the  outer  border  of  each  eyeball  with  the  fingers.  All 
1  Le  Conte,  Sight,  p.  255. 


The  Sense  of  Sight  175 

objects  in  view  are  now  seen  double,  and  if  the  right  eye  be 
shut  the  left  image  disappears,  and  vice  versa. 

How  an  Object  is  seen  as  One  with  Two  Eyes. — 
When  we  look  at  an  object  in  the  far  distance  the  antero- 
posterior axes  of  the  eyes  are  parallel,  and  an  image  of  the 
object  will  be  formed  upon  the  spot  of  distinct  vision  in 
each  eye.  Again,  when  the  object  viewed  is  near  at  hand,  the 
visual  axes  converge,  so  that  the  image  is  still  formed  upon 
the  yellow  spot  of  both  eyes,  and  the  object  is  seen  as 
single.  This  sensation  of  oneness  arises  from  the  habitual 
use  of  these  areas  of  the  retinse  for  the  observation  of  one 
and  the  same  point,  and  from  the  attention  given  to  that 
point  alone  as  distinguished  from  all  others  in  the  visual 
field.  But  if  we  displace  one  of  the  visual  axes  by  pressing 
with  the  finger  upon  the  corresponding  eye  we  will  seem  to 
see  all  objects  doubled,  one  image  being  stationary,  the 
other  moving  as  we  vary  the  pressure.  The  reason  for  this 
is  as  follows  :  under  ordinary  circumstances  the  mind  pro- 
jects the  image  formed  in  the  eye  outwards  in  the  direc- 
tion of  the  visual  axis,  and  this  being  now  mechanically 
displaced  the  object  seems  to  be  in  motion. 

But,  further,  since  the  whole  field  of  normal  vision  seems 
single  when  seen  with  both  eyes,  it  follows  that  the  retinas, 
as  a  whole,  act  in  combination,  and  give  a  single  image  of 
that  which  is  focussed  upon  them.  Now,  suppose  we  hold 
two  pencils  upright  in  the  middle  plane  of  the  body,  but  at 
different  distances,  we  can  voluntarily  fix  our  attention  upon 
one  or  other,  and  the  one  upon  which  we  concentrate  our 
regard  will  appear  single,  while  the  other  will  be  indistinctly 
seen  and  will  seem  double.  The  image  of  that  one  to 
which  we  specially  attend  is  single  because  the  visual  axes 
converge  upon  it,  but  the  other  is  indistinct  and  double 
because  its  images  on  the  two  retinas  are  not  in  the 
line  of  regard,  and  not  upon  points  which  habitually  act 


176 


Physiology  of  the  Senses 


together.  For  each  person  there  is  always  a  certain  visual 
field,  determined  in  shape  by  the  outlines  of  the  eyebrows, 
nose,  and  cheeks,  and  by  the  position  of  the  eyes  in  regard 
to  them,  a  field  from  each  point  of  which  rays  entering  the 
eyes  always  fall  upon  corresponding  points  in  the  two  eyes. 


/ 


7/ 


Fig.  82. — Binocular  visual  field.  If  a  sheet  of  paper  be  held  so  as  to  touch  the 
brow  and  prominence  of  the  nose,  the  binocular  visual  field  will  be  seen  as  in 
the  space  in  I,  bounded  by  the  lines  L  and  R.  If  the  paper  be  held  a  few 
inches  from  the  face  the  area  visible  to  both  eyes  will  have  the  shape  seen 
in  II. 


If,  the  head  being  fixed  and  both  eyes  open,  the  extent  of 
the  whole  visual  field  be  noted,  and  if  the  right  and  left 
eyes  be  alternately  closed  and  opened,  it  will  be  found  that 
the  projection  of  the  eyebrows  and  nose  cuts  off  from  each 
eye  a  certain  part  of  the  visual  field  which  is  visible  to  the 


The  Sense  of  Sight 


177 


other  eye,  and  that  there  is  a  central  area  common  to  both 

eyes,   or  a  binocular  visual  field,   shaped   as   in   Fig.   82. 

This  area  bears  a  fixed  form  and  magnitude,  and  from  it 

alone  can  rays  of  light  enter  both  eyes.      From  each  point 

in  this  field  the  rays  of  light  entering  the  eyes  must,  for  a 

given  state  of  accommodation,  fall  upon  the  same  points  of 

the  retinae.     To  each  point,  then,  in  the  binocular  visual 

field  there  is  a  corresponding  point  in  each  retina  ;  and, 

again,  the  right  side  of  the  right  retina  corresponds  point 

for  point  with  the  right  side  of  the  left  retina,  and,  similarly, 

the  left  side  of  the  right  retina 

corresponds  with  the  left  side 

of  the  left  retina.      Thus  it 

follows  that  the  upper  halves 

correspond,  and  likewise  the 

lower.       The    yellow    spots 

form     corresponding    areas, 

and  when  the   images  of  a 

small    object    formed    upon 

these,  and  projected  outwards 

by  the  mind  upon  the  visual 

field,  coincide  in  position  the 

object  is  seen  single. 

If,  for  example,  the  eyes  are 
so  directed  that  the  images 
upon  them  of  the  point  A  (Fig.  83)  are  projected  outwards 
so  that  the  lines  of  projection  meet  at  A,  we  will  see  A  as 
one  point,  and  any  other  point  in  its  near  vicinity,  such  as 
B,  will  likewise  be  seen  single,  because  its  images  are 
formed  upon  corresponding  points  of  the  retinae.  If  we 
describe  a  circle  whose  circumference  passes  through  the 
point  of  sight  and  the  two  optic  centres,  it  may  be  mathe- 
matically shown  that  rays  from  all  points  of  this  circle  fall 
upon    corresponding  points,   and    objects    on    it   are   seen 

N 


Fig.  83. — Diagram  of  one  form  of 
horopter.     (Miiller.) 


i78 


Physiology  of  the  Senses 


single.  M tiller  called  this  circle  the  horopter',  and,  for 
different  positions  of  the  eyes,  the  horopter  may  assume 
complicated  forms,  but  in  any  horopter  all  points  are  seen 
single. 

We  are  now  able  to  understand  how  a  double  image  is 
seen  when  objects  not  in  the  horopter  are  seen  double. 
Suppose  in  the  case  of  looking  at  the  pencils  we  represent 
the  nearer  one  by  p  (Fig.  84),  the  farther  by  p '.  Then, 
when  the  eyes  are  converged  on  /,  the  images  of  p'  are  not 


b  ~K 

Fig.  84. — Diagram  to  illustrate  formation  of  homonomous  double  images. 

formed  on  corresponding  points  of  the  retinae,  but  are  each 
to  the  mner  side  of  the  yellow  spot  at  bb\  and  two  faint 
images  of  p'  are  seen,  one  on  each  side  of,  and  at  the  same 
distance  from,  the  eyes  as  p,  viz.  for  the  left  eye  at  a,  for  the 
right  eye  at  a'.  On  shutting  one  or  other  eye,  the  image 
on  the  same  side  disappears,  and  it  is  said  to  be  homo- 
nomous.  But  if  the  gaze  be  fixed  upon  p'  (Fig.  85)  a 
double  image  of  p,  formed  external  to  the  yellow  spot  on 
both  eyes,  is  mentally  projected  outwards  to  the  distance  of 
the  plane  da  through  /,  and  now  on  shutting  one  or  other 


The  Sense  of  Sight 


179 


eye  the  image  on  the  opposite  side  disappears,  and  it  is 
hence  said  to  be  heteronomons. 

Now,  as  a  rule,  we  are  not  conscious  of  the  formation  on 
the  retina,  nor  does  the  mind  project  outwards  this  double 
image.  It  is  only  by  special  attention  to  the  action  of 
both  eyes  that  we  become  conscious  of  it ;  and,  at  a  first 
attempt,  it  is  sometimes  difficult  to  convince  a  person  that  a 
double  image  is,  as  in  the  above  experiment,  visible.  The 
reason  of  this  is,  that  attention  is  paid  to  the  object  directly 
looked  at  and  not  to  the  fainter  double  images  ;  and  also 


Fig.  85. — Diagram  to  illustrate  formation  of  heteronomous  double  images. 

because  where  we  do  try  to  see  two  objects  at  different 
distances  at  one  and  the  same  time,  the  minds  of  most 
people  attend  only  to  the  image  formed  by  the  right  eye 
and  disregard  that  of  the  left.  Thus,  if  you  tell  a  person 
to  point  with  the  finger  at  a  distant  object,  both  eyes  being 
open,  and  then  ask  him,  while  holding  the  hand  steadily,  to 
shut  the  right  eye,  he  will  seem  to  be  pointing  to  the  right 
of  the  object,  and  not  directly  at  it ;  but  if  he  shuts  his  left 
eye  he  will  seem  to  be  pointing  correctly.  This  applies 
more  especially  to  right-handed  persons,  the  reverse  being 


180  Physiology  of  the  Senses 

the  case  with  those  who  are  left-handed.  By  careful 
observation,  we  can  note  the  two  images  of  the  finger 
pointing,  and  may  bring  the  more  distant  object  between 
the  images,  and  then,  whether  the  right  or  left  eye  be  shut, 
the  finger  will  not  seem  to  be  pointing  directly  at  the  dis- 
tant point.  Still  another  reason  why  we  neglect  double 
images  is  that  these  are  often  so  large  as  to  overlap  one 
another,  and  so  be  practically  indistinguishable  ;  and  the 
effect  of  the  two  combined  in  a  psychical  process  by  the 
mind  is  to  lead  to  the  perception  of  the  third  dimension  in 
space,  or  in  other  words,  the  perception  of  solidity. 

Perception  of  Solidity.  —  When  we  look  at  a  solid 
body  the  images  formed  in  the  two  eyes  are  not  exactly 
the  same,  because  the  right  and  left  eyes  view  it  from 
different  standpoints.  This  can  be  best  appreciated  by 
viewing  some  small  object  at  no  great  distance  from  the 
eye,  e.g.  a  book.  If  we  alternately  examine  the  book  with 
the  right  and  left  eye,  the  other  being  meanwhile  closed, 
and  compare  mentally  the  appearances  presented  to  the 
two  eyes,  we  observe  that  the  right  eye  sees  more  of  the 
right  side  of  the  book,  the  left  more  of  the  left.  If  we  then 
note  what  area  of  background  is  hidden  by  the  two  images, 
we  find  that  the  .part  hidden  from  the  right  eye  by  the  book 
is  different  from  that  for  the  left.  Now,  with  both  eyes 
open,  let  vision  be  accommodated  for  the  background,  but 
examine  the  effect  produced  by  the  interposition  of  the 
book.  We  are  then  conscious  of  a  solid  opaque  body 
obscuring  part  of  the  background  completely,  while  to 
either  side  of  this  is  a  spectral  transparent  image  of  the 
sides  of  the  book  through  which  the  wall  seems  to  be  seen. 
On  shutting  the  left  eye  the  solid  body  seems  to  move 
to  the  left,  rendering  the  left  spectral  part  opaque,  because 
the  part  of  the  wall  formerly  seen  by  the  left  eye  is  no 
longer  visible,  and  similarly  for  the  right.      It  will  further 


The  Sense  of  Sight 


181 


be  noted,  as  we  converge  the  eyes  on  the  book,  that  the 
spectral  parts  disappear,  and  we  see  the  one  solid  body 
only.  Lastly,  if  we  look  at  the  book  fixedly  for  some  time, 
one  eye  being  shut,  and  then  if  we  look  with  both  eyes,  it 
is  at  once  seen  that  the  book  stands  out  in  much  bolder 
relief,  the  various  sides  and  borders  taking  their  natural 
inclination  in  reference  to  space.  A  suitable  object  for  the 
study  of  this  phenomenon  is  a  truncated  pyramid  upon  which 
we  look  vertically  downward.  With  both  eyes  open  the 
appearance  presented  is  that  seen  in  B  (Fig.  86).  Keeping 
the  head  in  the  same  position,  but  looking  with  the  left  eye 


X 


Y      X 


\ 

X 

y 

z 

\ 

z 


L 


Fig.  86. — Appearance  of  a  truncated  cone  seen  from  above  with  B,  both  eyes, 
L,  left  eye,  or  R,  right  eye. 


only,  we  will  see  the  cone  as  in  L,  or  with  the  right  eye  only, 
as  in  R. 

The  Stereoscope. — The  combination  of  L  and  R,  so 
as  to  give  the  appearance  of  solidity  to  the  eye,  may  be 
made  by  the  ste?-eoscofte,  an  instrument  invented  by  Wheat- 
stone,  who  first  noticed  that  the  perception  of  solidity  was 
due  to  the  dissimilarity  of  the  images  presented  to  the 
retinae.  In  its  simplest  form  the  reflecting  stereoscope 
consists  of  two  mirrors  placed  at  right  angles  to  each  other, 
as  in  Fig.  87.  The  eyes,  looking  into  these  obliquely,  see 
reflections  of  the  dissimilar  figures  R  and  L  representing 
the  appearances   as   seen  by   each   eye   individually ;    and 


1 82  Physiology  of  the  Senses 

the  images,  mentally  projected  backwards  in  the  line  of 
vision,  are  combined  at  the  point  of  intersection  of  the 
optic  axes,  and  we  seem  to  see  the  single  solid  object  as 
we  would  if  we  were  looking  at  it  with  both  eyes. 

Brewster's  refracting  stereoscope  is  the  one  in  common 
use.  In  this  instrument  the  optical  effect  is  obtained  by 
means  of  two  lenses  so  arranged  that  rays  of  light  passing 
from  the  stereoscopic  pictures  impinge  on  the  retina,  and 
are  projected  backwards  so  as  to  converge  and  meet  at  points 


Fig.  87. — Wheatstone's  stereoscope. 

behind  the  plane  of  the  pictures,  as  in  Fig.  88.  Each  eye 
thus  sees  its  own  picture,  but  corresponding  points  are 
brought  to  a  focus,  and  in  the  union  of  all  we  have  one 
picture  in  relief. 

The  apparently  differing  distances  from  the  eye  of 
different  parts  of  the  combined  picture  are  due  to  the 
differing  distances  between  corresponding  points  of  the 
constituent  pictures.  Those  pairs  of  points  which  are 
nearest  together  stand  out  in  highest  relief,  or  in  other 
words,  require  the  greatest  convergence  of  the  optic  axes, 
while  those  which  are  most  distant  from  one  another  seem 


The  Sense  of  Sight 


183 


most  remote  in  the  combined  picture.      In  Fig.  86,  p.  181, 

X   Y   Z 

the  points  v,'     '     ,  are  respectively  at  equal  distances  from 

one   another,  and   consequently   seem   to  be   in   the   same 

plane  in  B.      Similarly    ,'-7    ',   are  at  equal  distances  from 

x ,  y  ,  z , 

one  another,  and  seem  to  be  all  in  one  plane,  but  the  dis- 


FiG.  88. — Diagram  illustrating  the  principle  of  Brewster's  stereoscope.  The 
points  x,  x  forming  images  x! ',  x'  are  projected  outwards  and  coincide  at  X  ; 
the  points  y,  y,  being  nearer  to  one  another  than  x,  x,  appear  to  coincide  at  a 
point  Y  in  a  plane  nearer  to  the  eyes  than  X.     (After  Landois  and  Stirling.) 

tance  between  any  pair  of  these  being  less  than  the  distance 

XYZ 

between    any    pair   of  the   set     ", '  v,'     ',     the    plane    xyz 

A  ,    Y  ,  L  , 

seems  nearer  than  the  plane  XYZ.  Hence  the  trun- 
cated apex  of  the  pyramid  seems  nearer  the  eye  than  the 
base.  But  if  we  transpose  R  and  L  so  that  R  is  opposite 
the    left   eye    and   L   opposite   the  right,  then   the    points 

"*"'  •%'     ,  will  respectively  be  farther  from  each  other  than 
x,y,z 

XYZ 

, '     '     '   and  we  seem  to  be  looking  into  a  hollow  pyramid, 
X  ,  Y  ,  Z, 


184  Physiology  of  the  Senses 

whose  apex  is  directed  away  from  us.  In  Fig.  88  the  points 
x,  x,  being  farther  apart  than  y,  y,  are  combined  at  X  in  a 
plane  behind  that  through  Y,  the  point  of  combination  of 

It  is  indeed  unnecessary  to  have  a  stereoscope  to  get 
the  combined  effect.  If  we  merely  fix  the  eyes  upon  the 
diagram,  but  accommodate  the  vision  for  distance,  we  will 
see  the  two  diagrams  apparently  moving  towards  each 
other  and  overlapping  until  they  seem  to  coincide,  when 
suddenly  the  effect  of  a  solid  body  between  two  faintly 
visible  flat  diagrams  is  perceived.  Ordinary  stereoscopic 
pictures  are  obtained  by  taking  photographs  of  the  same 
scene  from  slightly  different  standpoints,  corresponding  to 
the  distance  between  the  right  and  left  eyes.  These  are 
fixed  to  a  card  in  their  proper  relationship  to  the  right  and 
left  eye  ;  and  if  reversed,  they  give  an  inverted  picture, 
all  solid  bodies  seeming  to  be  hollow.  Even  with  the 
pictures  properly  placed  it  is  possible,  by  a  simple  arrange- 
ment of  lenses,  as  in  the  instrument  called  the  ftsendoscope, 
to  displace  the  picture  so  that  our  judgment  of  the  size  of 
objects  is  disturbed  by  the  apparent  alteration  in  their 
distance  from  us. 

The  Telestereoscope. — The  stereoscopic  effect  depend- 
ing upon  the  distance  between  the  eyes,  it  will  naturally 
be  greater,  the  greater  the  distance.  We  cannot,  indeed, 
increase  the  distance  between  the  eyes,  although  a  small 
solid  body  stands  out  in  higher  relief  when  near,  the  eyes 
than  when  far  away,  because  the  visual  axes  are  more 
convergent.  But  von  Helmholtz  has  invented  an  ingenious 
instrument  by  which  the  eyes  are  virtually  separate  and 
a  more  powerful  stereoscopic  effect  obtained.  It  is  known 
as  the  ftsendoscope  or  telestereoscope,  and  the  principle  of  its 
construction  is  as  follows.  Two  mirrors  are  placed  parallel 
and  a  little  to  the  side  of  the  mirrors  used  in  Wheatstone's 


The  Sense  of  Sight  185 

stereoscope  (Fig.  89).  The  rays  from  the  object  to  the 
outer  mirrors  are  reflected  to  the  inner  mirrors,  and  thence 
to  the  eyes.  It  thus  happens  that  rays  falling  on  mirrors 
much  more  distant  from  each  other  than  the  eyes,  enter 
the  eyes  as  if  coming  directly  to  them  from  the  object. 
We  are  thus  able  to  see,  as  it  were,  more  of  the  sides  of 
the  body  than  we  could  under  ordinary  circumstances  ; 
distant  objects  seem  to  be  brought  nearer,  judging  by  their 
greater  relief,  and  all  parts  of  the  field  likewise  stand  out 
in  a  more  marked  manner  than  usual. 

In  viewing  the  different  parts  of  a  solid  body,  or  the 
apparently  nearer  and  more  remote  parts  of  a  stereoscopic 
picture,  there  is  a  constant 
movement  of  convergence  or 
divergence  of  the  eyes,  and 
hence  it  was  maintained  that 
a  prime  factor  in  the  percep- 
tion of  solidity  is  the  sense 
of  muscular  effort  required  in 
moving  the  eyes  from  point 

tO  point.       This  theory,  how-  FlG>  ^.-Telestereoscope      For 

r  J '  explanation,  see  text. 

ever,  is  negatived  by  the  fact 

that  we  have  quite  a  correct  perception  of  the  spatial 
relations  of  objects  when  seen  by  the  instantaneous  flash 
of  lightning,  a  flash  which  takes  place  so  rapidly  that 
there  is  no  time  for  all  the  complicated  processes  involved 
in  muscular  action.  Similarly,  the  stereoscopic  effect  is 
seen  when  the  picture  is  seen  by  the  light  of  the  electric 
spark  ;  that  is  to  say,  in  a  time  not  exceeding  the  o4q00- 
part  of  a  second.  But  though  the  time  of  stimulation  of 
the  retina  is  momentary,  there  is  an  appreciable  time  lost 
in  the  physical  change  of  the  condition  of  the  retina,  in  the 
passage  of  the  nerve  current,  in  the  arousing  of  sensation 
and    the    fusion    of    the    stimuli.     Wheatstone   held   that. 


1 86  Physiology  of  the  Senses 

in  the  fusion  of  two  images  not  mathematically  similar, 
the  mind  superadds  the  perception  of  solidity.  If  the 
points  in  the  two  pictures  are  so  far  apart  that  the  con- 
verging apparatus  is  unable  to  bring  them  to  a  focus,  we 
only  see  two  flat  pictures.  If  the  two  pictures  are  exactly 
similar,  and  their  points  may  be  exactly  fused,  the  result 
is  a  flat  picture.  The  mental  fusion  is  the  cause  of  the 
new  sensation.  The  fusion  in  ordinary  circumstances  is  to 
all  intents  and  purposes  complete.  The  external  world 
presents  itself  to  us  with  each  object  clearly  single  and 
defined.  It  is  only  when  we  pay  close  attention  and 
carefully  analyse  our  visual  sensations  that  we  can  detect 
the  fact  of  incomplete  fusion. 

We  have,  for  example,  the  sensation  of  luminosity. 
When  carefully  examined  this  is  found  to  be  due  to  the 
irregular  reflection  of  rays  of  light  from  the  uneven  surface 
of  a  body ;  calm  water  is  non -luminous,  rippling  water 
sparkles  with  light,  but  the  amount  of  light  going  from  the 
broken  surface  to  one  eye  differs  from  that  going  to  the 
other,  and  the  effort  at  fusion  of  the  darker  and  the  lighter 
gives  rise  to  the  sensation  of  luminosity.  The  combined 
stereoscopic  picture  is  luminous  from  the  superposition  of 
darker  and  lighter  spots  in  the  one  picture,  or  the  reverse 
in  the  other.  And  yet  the  fusion  is  incomplete  when  we 
look  into  the  matter  closely.  By  an  effort  of  will  we  can 
allow  the  dark  or  the  light  to  preponderate.  Suppose 
we  have  two  stereoscopic  pictures,  as  in  Fig.  90,  one 
of  which  is  light  on  a  dark  ground,  the  other  dark  on  a 
light  ground,  we  can,  by  a  voluntary  effort,  superpose  the 
one  over  the  other  and  give  rise  to  the  impression  of  a 
luminous  solid  body ;  but  we  can  also  easily  alter  the 
depth  of  the  grayish  luminosity  by  paying  attention  to  the 
dark  or  the  light  picture  at  will. 

We    have    here,    indeed,    an    analogy   to   the   detection 


The  Sense  of  Sight 


187 


by  the  ear  of  the  elements  of  a  compound  tone.  The 
practised  ear  is  able  to  separate  and  attend  to  any  one 
elementary  tone,  or,  on  a  larger  scale,  to  any  individual 
instrument  in  an  orchestra ;  and  the  mind  may  dwell  only 
on  the  harmonious  fusion,  experiencing  a  pleasure  from  the 
combination,  or  it  may  give  itself  up  at  will  to  the  effect  of 
one  or  of  all.  The  process  is  easier  with  the  ear  than  with 
the  eye.  The  optical  fusion  is  more  complete,  more  diffi- 
cult to  analyse.  But  it  may  be  made  easier  if  we  endeavour 
to  fuse  two  surfaces  of  different  colours  in  the  stereoscope. 
Here  there  is  not  complete  mixing  of  the  colours,  but  the 
colour  sensation  is  now  that  of  one,  now  that  of  the  other 


Fig.  90. — Diagram  to  illustrate  causation  of  sensation  of  luminosity. 

colour,  the  varying  effect  being  probably  due  to  changes  in 
the  activity  of  the  two  retinae. 

Estimation  of  Distance. — The  foregoing  considerations 
on  the  perception  of  solidity  will  assist  us  in  answering 
the  more  general  question  as  to  the  estimation  of  space  07' 
distance.  We  have  seen  that  the  muscular  effort  at  con- 
vergence is  greater  for  near  than  for  remote  objects,  and 
the  greater  the  effort  experienced  the  nearer  do  we  judge 
the  object  to  be.  But  accompanying  the  effort  at  converg- 
ence there  is  usually  a  muscular  action  of  accommodation. 
The  pupil  contracts  to  shut  off  divergent  rays  of  light  which 
would  cause  blurring  of  the  image,  and  the  ciliary  muscle 
contracts  in  order  to  lengthen  the  focal  distance  of  the  eye 


1 88  Physiology  of  the  Senses 

for  the  nearer  object.  Each  of  these  muscular  efforts  must 
add  its  quantum  to  the  general  sum  of  muscular  sensation. 
Objects  at  the  point  of  sight  are  seen  in  clear  detail,  while 
those  which  are  nearer  or  farther  off  are  seen  indistinctly, 
and  we  unconsciously  judge  of  differing  distances  by  varying 
efforts  of  accommodation.  The  dimness  of  bodies  within 
the  near  point  of  vision  is  due  to  the  impossibility  of  focuss- 
ing the  object.  Far-distant  objects  are  dimly  seen  because 
of  the  aerial  perspective.  The  atmosphere  not  being  per- 
fectly transparent  and  colourless,  small  details  are  blotted 
out,  and  variety  of  colour  lost  in  a  bluish  haze.  The  dis- 
tant parts  of  a  landscape  are  conceived  to  be  nearer  and 
smaller  when  seen  in  wet  weather  than  in  dry,  for  dust- 
laden  air  gives  a  more  marked  aerial  perspective  than  that 
which  has  been  washed  by  rain ;  and  again,  in  misty 
weather  the  half-hidden  forms  of  men  may  seem  far  away 
and  of  supernatural  size. 

Again,  varying  convergence  assists  our  estimation  of 
distance,  not  only  through  the  muscular  effort  involved, 
but  also  by  variation  of  the  angle  of  convergence  of  the 
visual  axes  upon  the  object.  For  objects  of  similar  size 
it  is  evident  that  the  angle  of  convergence  must  be  greater 
for  near  than  for  remote  objects.  We  learn  through  the 
other  senses,  as  well  as  through  sight,  to  know  the  com- 
parative sizes  of  objects,  and  by  noting  and  comparing  the 
apparent  size  of  objects  we  arrive  at  a  judgment  as  to  their 
distance,  the  seemingly  smaller,  of  course,  being  considered 
the  more  distant.  Persons  who  have  lost  the  use  of  one 
eye,  and  therefore  the  valuable  aid  of  convergence,  cannot 
judge  accurately  of  the  distance  of  near  objects.  If  asked 
to  touch  an  object  quickly  they  are  apt  to  fall  short,  as  ex- 
perience tells  them  they  may  misjudge  and  strike  it  roughly 
if  they  attempt  to  reach  the  full  apparent  distance. 

Estimation  of  distance  is  likewise  assisted  by  observation 


The  Sense  of  Sight 


189 


of  the  distance  of  the  background  over  which  a  body  near 
to  the  eye  seems  to  move  when  the  relative  positions  of  the 
eye  and  the  body  are  changed. 

In  Fig.  91,  I.  the  eye  E  moves  while  the  body  B  is 
stationary,  in  II.  the  body  moves  from  B  to  B'  while  the 
eye  is  stationary.  The  apparent  distance  moved  by  B  upon 
XY  is  only  ad,  while  upon  X'Y'  it  is  the  much  larger 
distance  db' .  The  distance  over  which  the  body  seems  to 
pass  gives  an  indication  of  the  relative  distances  of  the 
planes  XY,  X'Y'  from  the  observer. 

We  are  also  able  to  give  a  more  accurate  estimate  of 


Fig.  91. — Estimation  of  distance  from  change  in  relative  position  of  the  eye  and 
of  an  object  observed. 

the  distance  between  two  points  when  several  objects 
intervene.  We  take  a  series  of  mental  leaps,  as  it  were, 
from  point  to  point,  the  effort  of  which  is  greater  than  that 
of  passing  over  the  whole  distance  at  one  effort.  The  dis- 
tance between  A  and  B   (Fig.  92)  seems  greater  than  that 

between  B  and  C  oh  account 

A  £  C 

of  the  intervening  dots,  but  it    •••••••  • 

is  the  same.      Children  often  FlG 

amuse    themselves   with   the 

following  experiment.     A  boy,  after  looking  at  a  landscape 

in   an   erect   posture,   will   turn,    stoop   down,  and  view  it 

between  his  legs,  and  all  objects  will  seem  farther  off,  as, 

from  the  unaccustomed  posture  and  the  proximity  of  the 


190 


Physiology  of  the  Senses 


head  to  the  ground,  objects  in  the  foreground,  formerly  dis- 
regarded, are  now  more  dwelt  upon.  Similarly,  the  sky 
seems  nearer  us  at  the  zenith  than  at  the  horizon,  and  a 
landsman  has  great  difficulty  of  judging  distances  at  sea. 
The  eye  projects  the  image  of  the  object  viewed  outwards, 
but  if  it  be  at  any  great  distance,  the  lines  of  projection 
from  the  two  eyes  are  practically  parallel,  and  judgments 
as  to  size  guide  the  judgment  as  to  distance.  It  is  interest- 
ing to  note,  in  this  regard,  that  persons  who  have  been  born 
blind  and  have  by  an  operation  gained  the  power  of  vision, 
seem  at  first  to  see  all  objects  close  to  the  eye  or  almost 
touching  it — they  "see  men  as  trees  walking" — and  it  is 
only  after  a  process   of  education  in  which  the   sense  of 


Fig.  93. — a  and  b  are  of  the  same  length,  but  b  subtends  a  greater  visual  angle, 
being  nearer  to  the  eye. 

touch  has  much  to  do  that  they  are  able  to  form  a  proper 
estimate  of  externality  or  distance  through  vision. 

Estimation  of  Size. — Closely  connected  with  our  esti- 
mate of  distance  is  that  of  size.  This  primarily  depends 
on  the  size  of  the  retinal  image,  or  in  other  words,  of  the 
visual  angle  subtended  by  the  object.  In  Fig.  72,  p.  146, 
X  is  the  visual  angle  subtended  by  the  lines  c,  d,  and  e,  and 
since  these  objects  make  a  retinal  image  of  the  same  size  it 
is  evident  that,  in  estimating  size,  it  is  necessary  to  have  at 
least  an  approximate  idea  of  the  distance  of  the  object  from 
the  eye.  The  moon  subtends  a  larger  visual  angle  than  the 
stars  because  it  is  so  much  nearer  to  us,  not  because  of  its 
greater  size. 

We  learn  by  experience,  more  especially  by  the  com- 


The  Sense  of  Sight 


191 


bination  of  touch  and  vision,  that  if  two  objects  of  different 
sizes  subtend  the  same  visual  angle,  the  nearer  of  the  two  is 
the  smaller ;  and  the  young  artist  measures  the  comparative 
length  and  breadth  of  distant  objects  by  holding  his  pencil 
at  arm's  length  between  his  eye  and  the  thing  sketched. 

The  degree  of  co7iverge?ue  of  the  visual  axes  is  also  of  much 
importance  in  the  estimation  of  size.  For  by  experience  we 
know  that  an  object  of  known  size  will  subtend  a  certain 
visual  angle  at  a  given  distance,  and  that  the  nearer  the 
object  is  to  the  eye  the  greater  will  be  the  angle  subtended, 
as  in  Fig.  93.      Then,  of  all  bodies  which  subtend  the  same 


A 


B 

Fig.  94. 


visual  angle,  that  one  must  be  the  largest  which  requires  the 
least  convergence. 

Thus,  too,  the  intervention  of  bodies  of  known  size  gives 
an  idea  as  to  the  size  of  the  more  remote  object.  The  sun 
seen  on  the  horizon  behind  trees  seems  larger  than  when  in 
mid-heaven,  because  we  have  a  better  estimate  of  its  dis- 
tance and  of  the  visual  angle  it  should  thus  subtend.  Few 
people  agree  in  their  estimate  of  the  apparent  diameter  of 
the  full  moon,  and  in  Fig.  94,  B  seems  to  have  the  greatest 
height  from  a  mental  summation  of  the  horizontal  spaces, 
A  the  greatest  breadth,  and  C  to  be  the  smallest.  Yet  all 
are  of  the  same  area.  In  this  case  the  three  figures  are  of 
the  same  size,  and  must  give  rise  to  retinal  images  of  the 
same  size,  but  the  basis  on  which  we  form  our  judgment  as 


192 


Physiology  of  the  Senses 


to  the  area  of  each  being  different,  we  judge  them  of 
different  size.  The  judgment  errs,  not  the  organs  of 
vision. 

This  error  of  judgment  is  perhaps  even  more  marked  in 
the  case  of  Fig.  95,  where  the  line  A  seems  longer  than  B, 
although   in   reality   of  the   same   length.      In   A   there   is 


A 


Fig.  95. 

insensibly  divergence  of  the  optic  axes,  in  B  there  is  con- 
vergence, owing  to  the  oblique  lines. 

The  illusion  is  somewhat  different,  but  it  is  also  marked 
in  Fig.  96,  known  as  Zollner's  lines.  Here  the  oblique 
lines  seem  to  converge 
towards  one  another, 
though  really  parallel. 
The  unconscious  ten- 
dency to  follow  the 
short  lines  till  they 
would  intersect  leads 
to  an  impression  that 
the  oblique  lines  would 
meet  if  produced  in 
the  opposite  direction. 

Allied  to  this  illu- 
sion of  vision  is  that 
produced  by  drawing 

a  thin  line  to  intersect  a  broad  line  obliquely. 
EF,  not  CD,  is  in  the  same  straight  line  as  AB. 

Illusions  of  Vision  also  arise  when  we  look  for  a  short 
time  at  a  body  in  motion  and  then  turn  our  eyes  upon 
one  at  rest.      It  seems  to  move  in  the  opposite  direction, 


Fig.  96. — Zollner's  lines. 

In  Fig.  97, 


The  Sense  of  Sight  193 

whether  that  has  been  one  of  rotation  or  of  movement  in  a 
straight  line.  Thus  if  we  gaze  for  about  a  minute  at  a  wheel 
revolving  rapidly  on  a  fixed  axis,  and  then  turn  our  eyes 
to  the  ground,  a  similar  area  seems  to  rotate  in  an  opposite 
direction  round  the  centre  of  vision.  Similarly,  as  stated 
on  p.  155,  when  upon  the  deck  of  a  ship  in  motion,  if  we 
look  for  a  time  on  the  water  and  then  at  the  deck,  some 
of  the  boards  seem  to  creep  forwards  relatively  to  those 
adjoining  them.  In  looking  at  the  water  we  instinctively 
try  to  fix  our  eyes  upon  points  in  the  seemingly  moving 
surface,  and  so  the  eyes  have  a  backward  movement.  Owing 
to  the  persistence  on 
the    retina    of   visual  v^ 

impressions,  we  con- 
tinue unconsciously  to 
seek  back  towards  the 
previously  vanishing 
point ;   and   in   doing 

so     the    new    image  j} 

created  by  the  body,  FlG>  97._For  description,  see  text, 

stationary  with  regard 

to  ourselves,  seems  to  be  that  of  a  body  in  motion  in  the 
opposite  direction. 

Vision  assists  in  the  perception  oj  motion  mainly  by  the 
change  of  position  of  the  retinal  image  of  the  moving  body, 
relatively  to  the  fixed  position  of  the  image  of  the  rest  of 
the  visual  field.  If  the  eyes  follow  the  moving  body,  then 
its  image  is  fixed  on  the  retina,  while  the  rest  of  the  visual 
field  changes  its  position.  By  the  rapidity  of  movement  of 
eye,  head,  or  body,  we  judge  as  to  the  rate  of  movement  of 
the  object.  We  can  form  no  idea,  through  vision,  as  to  the 
direction  of  motion,  unless  we  have  this  relative  movement 
of  the  various  parts  of  the  field.  Sitting  in  a  railway  train 
in  motion,  there  is  a  change  of  position  of  near  objects  as 

o 


194  Physiology  of  the  Senses 

regards  ourselves  and  the  background,  which  is  so  rapid 
that  we  almost  imagine  them  to  be  in  motion.  If  another 
train  passes  us  going  in  the  opposite  direction,  it  seems  to 
be  going  with  great  velocity,  because  we  assume  the  com- 
partment in  which  we  sit  to  be  stationary,  and  the  velocity 
of  our  own  movement  is  added  to  that  of  the  other  train. 
Similarly,  if  two  trains  are  standing  side  by  side  at  a  station, 
and  the  one  adjoining  us  begins  to  move,  we  imagine  that 
it  is  the  train  in  which  we  sit  that  is  moving  in  the  opposite 
direction,  because  we  are  by  habit  led  to  believe  that  the 
station  with  all  its  contents  is  fixed,  while  our  train  is  the 
only  movable  body.  We  can  thus  enjoy  the  sensation  of 
somewhat  rapid  motion  without  the  jarring  that  usually 
accompanies  railway  travelling,  until,  the  other  train  having 
swept  past,  we  see  the  sides  of  the  station  beyond  silent 
and  motionless  ;  and  immediately  we  are  brought  to  rest 
by  a  more  smoothly  working  brake  than  has  yet  come  into 
general  use. 

Our  notions  of  the  form  of  objects  are  based  partly  on 
the  fusion  of  stimuli  of  different  parts  of  the  retina,  giving 
rise  to  a  sense  of  continuity,  and  partly  from  movements  of 
the  eyes  from  point  to  point.  The  body  may  be  a  plane 
figure  in  which,  owing  to  the  mode  of  construction,  we  may 
at  will  imagine  different  shapes  to  be  represented.  Fig.  98, 
for  example,  may  be  conceived  at  will  to  represent  either 
"a  staircase  against  a  wall,  or  an  overhanging  portion  of  a 
wall,  the  lower  part  of  which  has  been  removed,  and  whose 
under  surface  has  taken  the  form  of  steps."  1  In  the  former 
case,  we  regard  ab  as  running  backwards  from  a,  the  nearer 
point ;  in  the  latter,  we  suppose  b  to  be  the  nearer  point, 
and  a  the  more  remote,  and  run  the  eye  along  ab  in  the 
direction  from  b  to  a. 

In  the  perception  of  solidity  of  bodies,  the  possession,  as 

1  Bernstein,  The  Five  Senses,  p.  160. 


The  Sense  of  Sight 


i95 


we  have  seen,  of  binocular  vision  is  of  marked  advantage. 
The  movements  of  accommodation  and  convergence,  the 
wider  movements  of  the  whole 
eye  from  point  to  point  and 
from  plane  to  plane,  the  play 
of  light  and  shade,  the  relation 
to  surrounding  bodies  —  all 
these  are  factors  which  in- 
fluence the  mind  in  its  judg- 
ment as  to  solidity.  Nay, 
further,  in  certain  disordered 
conditions  of  the  brain,  old 
impressions  may  be  renewed 
and  recombined,  and  the 
surrounding  space  becomes 
peopled  with  fantastic  forms, 
lovely  or  terrible,  according 
to  mood — forms  as  real  and 
substantial    to    the   disturbed 


Fig. 


5. — For  description,  see  text. 
(After  Bernstein.) 


mind   as   those  which   appear   in   ordinary  vision.       How 
forcibly  has    this    been    painted    in  the  dagger    scene    in 

Macbeth — 


Is  this  a  dagger  which  I  see  before  me, 

The  handle  toward  my  hand  ?     Come,  let  me  clutch  thee. 

I  have  thee  not,  and  yet  I  see  thee  still. 

Art  thou  not,  fatal  vision,  sensible 

To  feeling  as  to  sight  ?  or  art  thou  but 

A  dagger  of  the  mind,  a  false  creation, 

Proceeding  from  the  heat -oppressed  brain  ? 

I  see  thee  yet,  in  form  as  palpable 

As  this  which  now  I  draw. 

Thou  marshall'st  me  the  way  that  I  was  going ; 

And  such  an  instrument  I  was  to  use. 

Mine  eyes  are  made  the  fools  o'  the  other  senses, 

Or  else  worth  all  the  rest ;  I  see  thee  still, 


196  Physiology  of  the  Senses 

And  on  thy  blade  and  dudgeon  gouts  of  blood, 
Which  was  not  so  before.      There's  no  such  thing: 
It  is  the  bloody  business  which  informs 
Thus  to  mine  eyes.1 

And  as  the  perturbed  mind  may  wander  in  an  illusory 
world  of  its  own,  so  the  abstracted  mind  may  disregard  the 
promptings  of  sense.  The  eye  is  open,  the  image  is  painted 
on  the  retina,  and  the  nerve  currents  pass  to  the  visual 
centre  ;  but  the  centre  is  preoccupied,  the  mind  goes  on 
its  own  way,  the  vision  is  unheeded.  Such  is  the  con- 
dition with  the  somnambulist.  He  rises  and  walks  in  his 
sleep  ;  his  eyes  are  open,  but  he  sees  only  that  which  fits 
in  with  his  dream.  So  it  is  with  the  mesmerised  man.  His 
mind,  otherwise  a  blank,  is  moved  this  way  and  that  at  the 
suggestion  of  the  operator,  and  in  a  stick  he  sees  a  hissing 
serpent,  or  an  empty  table  becomes  covered  with  choicest 
viands. 

Again,  as  vision  is  only  possible  so  far  as  the  visual 
apparatus  is  perfect,  and  since  we  find  the  organ  of  vision 
in  every  stage  of  advancement,  from  the  colour  spot  of  the 
invertebrate  up  to  the  complete  binocular  vision  of  man,  so 
we  may  infer  that  the  higher  intelligence  of  man  is  intimately 
associated  with  the  perfection  of  the  eye.  Crystalline  in  its 
transparency,  sensitive  in  receptivity,  delicate  in  its  adjust- 
ments, quick  in  its  motions,  the  eye  is  a  fitting  servant  for 
the  eager  soul,  and,  at  times,  the  truest  interpreter  between 
man  and  man  of  the  spirit's  inmost  workings.  The  rain- 
bow's vivid  hues  and  the  pallor  of  the  lily,  the  fair  crea- 
tions of  art   and  the   glance   of   mutual   affection,  all  are 

1  Macbeth,  Act  II.  Scene  i.  In  this  scene,  also,  the  great  dramatist 
pictures,  with  profound  psychological  insight,  the  struggle  between  the 
delusions  of  the  mind,  as  projected  into  space,  and  their  correct  appre- 
ciation by  the  reasoning  faculties.  The  words  indicating  the  applica- 
tion of  the  reason  are  printed  in  italics. 


The  Sense  of  Sight  197 

pictured  in  its  translucent  depths,  and  transformed  and 
glorified  by  the  mind  within.  Banish  vision,  and  the 
material  universe  shrinks  for  us  to  that  which  we  may 
touch  ;  sight  alone  sets  us  free  to  pierce  the  limitless  abyss 
of  space. 


SOUND  AND  HEARING 

The  organ  of  hearing  is  the  ear ;  but  the  human  ear  is  a 
much  more  complicated  apparatus  than  most  people  suppose. 
The  really  sensitive  part  of  the  ear,  the  part  in  which  the 
auditory  nerve  terminates,  and  where  physical  give  rise  to 
physiological  changes,  is  buried  deep  out  of  sight  in  the 
bones  of  the  cranium,  and  the  external  ear,  that  which  is 
seen  upon  the  outside  of  the  head,  forms  a  part  only  of  an 
elaborate  arrangement  whereby  sound  waves  may  be  trans- 
mitted inwards  to  the  true  end  organ  of  hearing.  But  while 
this  is  the  case  in  man,  in  many  of  the  lower  organisms  we 
find  an  ear  which  closely  resembles  the  human  ear  in  prin- 
ciple, though  much  simplified  in  detail,  and  situated  upon  or 
immediately  below  the  surface.  In  its  simplest  form  the 
ear  consists  of  a  set  of  cells  to  which  we  find  attached 
delicate  hairs  or  rod-like  structures,  which  are  thrown  into 
vibration  by  sound  waves.  These  cells  are  connected,  or 
are  in  apposition,  with  the  terminal  fibrils  of  the  auditory 
nerve  ;  and  when  agitated  by  sound  they  produce  a  nerve 
impulse,  which  in  turn  excites  the  central  nerve  cells, 
and  sound  is  heard.  The  first  step  in  complexity  of  organ- 
isation of  the  ear  is  that  the  hair  cells  are  no  longer  on  the 
free  surface,  but  line  in  part  a  membranous  sac  containing 
fluid,  the  cells  having  sunk  down  into  the  substance  of  the 
animal's  body,  and  being  thus  better  protected  from  injury 


Sound  and  Hearing 


199 


(Fig.  99).  The  sac  may  be  of  a  simple  globular  shape,  or, 
in  highly  developed  animals,  it  may  assume  a  very  com- 
plicated form  ;  so  much  is  this  the  case  in  man,  that  it  is 
known  as  the  membranous  labyrinth.  The  structure  of  the 
labyrinth  is,  as  we  shall  see,  of  a  most  delicate  and  elaborate 
nature,  and  though  in  the  embryonic  condition  it  is  near 
the  surface  of  the  head,  in  the  adult  it  is  at  least  \\  inch 
from  the  surface,  and  enclosed  in  bone  so  hard  that  it  is 
called  the  petrous  or  stony  bone.  The  osseous  covering 
coincides  to  a  great  extent  with  the  membranous  bag  inside, 
but  a  small  amount  of 
fluid  separates  the  sac 
from  its  walls,  and 
protects  it  from  rude 
shocks  transmitted 
through  the  bone. 
The  auditory  cells  are 
situated  in  certain 
parts  of  this  sac,  and 
the  auditory  nerve 
passes  to  them  through 
channels  in  the  bone. 
There    are    also    two 

openings  by  which  changes  of  pressure  may  be  transmitted 
from  without  to  the  fluid  surrounding,  and  that  contained  by, 
the  membranous  labyrinth.  But  these  openings  cannot  be 
seen  from  the  outside.  They  communicate  with  a  chamber 
known  as  the  middle  part  of  the  ear,  or  simply  the  middle  ear, 
or  tympanum,  or  drum,  a  chamber  containing  air  and  opening 
by  a  tube  passing  forwards  and  inwards  into  the  throat — 
the  Eustachian  tube.  The  middle  ear  is  separated  from  the 
passage  leading  to  the  auricle,  or  visible  ear,  by  a  mem- 
brane, known  as  the  membrana  tyjnpani  (or  drum-head), 
which  vibrates  in   response   to  sounds,  and   whose    move- 


FiG.  99. — Auditory  vesicle  of  Phialidium.  d\,  d<>, 
Epithelium  covering  the  sac  ;  k,  auditory  cells, 
with  hh  auditory  hairs  ;  np,  nervous  cushion  for 
the  auditory  cells,  connected  with  nr\,  the  lower 
nerve  ring.     (Hertwig  and  Lankester.) 


200  <        Physiology  of  the  Senses 

ments  are  communicated  to  a  chain  of  bones,  and  by  this 
chain  to  the  inner  ear.  The  membrana  tympani  closes  the 
passage  leading  inwards  from  the  outer  ear  or  auricle.  There 
are  thus  an  outer  and  middle  ear  for  the  collection  and 
transmission  of  sounds,  and  an  inner  ear  for  their  reception  as 
stimuli  of  sensation.  By  this  arrangement  the  ear  becomes 
more  sensitive,  for  the  middle  ear  acts  as  a  drum  giving 
resonance  and  strength  to  delicate  sounds  (Fig.  101). 

In  order  to  obtain  a  complete  understanding  of  the 
manner  in  which  sound  affects  the  ear,  we  must  consider 
carefully  the  structure  of  the  ear,  and  how  it  is  fitted  to 
respond  to  sonorous  vibrations. 

i.  External  Ear. — The  shape  of  the  external  ear  varies 
to  a  remarkable  degree,  and  in  some  of  the  lower  forms  of 
vertebrates  it  may  be  entirely  absent.  In  the  frog,  for 
example,  there  is  no  external  ear,  the  tympanic  membrane 
being  seen  as  a  disc  on  a  level,  and  continuous  with  the 
skin  of  the  head.  In  birds,  again,  the  auricle  is  absent, 
but  there  is  an  external  auditory  canal  or  meatus  leading 
down  to  a  membrana  tympani.  The  middle  and  internal 
ears  are  more  highly  developed  in  birds  than  in  reptiles, 
but  still  fall  far  short  of  the  human  ear  in  complexity. 
In  mammals,  the  auricle  is  of  very  varied  size  and  shape, 
and  it  may  be  either  stiff  and  erect  from  the  presence  of  an 
elastic  cartilage,  as  in  the  ear  of  the  horse  or  man,  or  it 
may  be  soft  and  yielding,  as  in  the  elephant.  The  surface  is 
usually  convoluted  and  funnel  or  trumpet  shaped,  so  as  to 
gather  the  waves  of  sound  to  the  best  advantage,  and  many 
animals  have  the  power  of  moving  the  opening  of  the  auricle, 
by  means  of  voluntary  muscles,  in  the  direction  from  which 
the  sound  comes.  Thus  the  horse  pricks  up  its  ears  when 
it  hears  a  sound,  and  no  doubt  its  appreciation  of  the  direc- 
tion of  sounds  is  thereby  assisted.  In  the  human  ear  there 
are  similar  voluntary  muscles,  but  man  has,  for  the  most 


Sound  and  Hearing 


201 


part,  ceased  to  have  the  power  of  moving  the  auricle  in 
response  to  sounds  from  varying  sources  apart  from  move- 
ments of  the  head  as  a  whole.  No  doubt,  by  attention  and 
practice,  a  man  may  acquire  the  power  of  moving  the 
auricle  slightly,  and  the  great  German  physiologist,  Muller, 
was  proud  of  being  able  to  do  so.  But,  at  best,  these 
movements  are  small  as  compared  with  those  of  the  lower 
animals.  Special  names  have  been  given  to  the  various 
depressions  and  protuberances  of  the 
auricle  (for  which  see  description  of 
Fig.  ioo). 

If  we  pass  the  finger  round  the 
border  of  the  ear  we  will  feel  near 
the  upper  part  a  small  nodule,  which 
is  interesting,  according  to  the  com- 
parative anatomists,  as  being  homo- 
logous with  the  tip  of  the  pointed  ear 
of  many  animals. 

The  general  effect  of  the  con- 
volutions of  the  surface  of  the  auricle 
is  to  collect  and  transmit  to  the  ex- 
ternal auditory  canal,  and  that  to  the 
best  advantage,  sound  waves  falling 
upon  the  surface  of  the  ear.  For 
just  as  waves  of  light  falling  upon  a 
transparent  body  are  partly  reflected  and  partly  trans- 
mitted, so  sound  waves  striking  the  auricle  are  partly 
concerned  in  giving  rise  to  corresponding  vibrations  in 
the  substance  of  the  auricle,  and  partly  reflected,  and  the 
more  the  waves  are  sent  to  the  inner  ear  the  more  intense 
will  be  the  sound.  The  phenomenon  familiar  to  every 
one,  of  the  echo,  is  an  example  of  this  reflection  of  sound 
on  a  large  scale  in  nature.  We  hear  first  the  sounds  trans- 
mitted directly  to  the  ear,  then  those  reflected  from  more 


Fig.  ioo. — Outer  surface  of  the 
right  auricle,  i,  Helix ;  2, 
fossa  of  helix  ;  3,  antihelix  ; 

4,  fossa    of   the    antihelix  ; 

5,  antitragus  ;  6,  tragus  ;  7, 
concha;  8,  lobule.    (Arnold.) 


202  Physiology  of  the  Senses 

or  less  distant  bodies.  In  the  whispering  gallery  of  St. 
Paul's  Cathedral  in  London,  or  in  the  ducal  mausoleum  at 
Hamilton,  faint  sounds  can  be  heard  at  a  considerable  dis- 
tance from  the  point  at  which  they  originate,  as  they  are 
reflected  in  such  a  way  as  to  be  focussed  at  a  special  point. 
So  the  shape  of  the  auricle,  by  focussing  sounds,  helps  the  ear 
to  hear  sounds  of  low  intensity.  It  would  appear  also  that 
the  form  and  size  of  the  depressions  of  the  concha  strengthen 
tones  of  very  high  pitch,  such  as  occur  in  hissing  sounds, 
like  the  noise  of  waves  breaking  on  a  shingle  beach,  or  that 
of  a  waterfall.  Thus  a  very  slight  change  in  these  depres- 
sions will  affect  the  musical  quality  of  tones.  If  the  irregu- 
larities of  the  surface  are  filled  with  wax,  sounds  are  not 
heard  so  loudly,  and,  conversely,  we  increase  our  receptivity 
by  putting  the  hand  to  the  ear,  and  turning  the  head  side- 
ways to  the  sound.  If  the  auricle  is  entirely  removed, 
hearing  is,  however,  but  little  diminished.  The  collecting 
power  of  the  auricles  assists  in  the  determination  of  the 
direction  from  which  a  sound  comes  ;  the  sound  being  more 
loudly  heard  in  one  ear  than  the  other,  we  conclude  that  it 
comes  towards  that  side  of  the  head  on  which  the  louder 
sound  is  heard. 

2.  Meatus  or  Passage. — From  the  pinna  or  auricle,  the 
external  auditory  meatus,  or  passage  to  the  middle  ear, 
passes  inwards  and  slightly  forwards,  being  inclined  at 
first  upwards  and  then  bending  downwards.  The  passage 
is  almost  circular  in  cross  section,  but  the  outer  end  is 
flattened  a  little  from  before  backwards,  while  the  inner 
part  is  broadest  in  the  horizontal  plane.  The  meatus  is 
closed  internally  by  the  tympanic  membrane,  or  drum-head 
(see  Fig.  101,  17),  which  lies  obliquely  to  the  direction  of 
the  lumen  of  the  tube,  the  lower  margin  being  farther  in 
than  the  upper,  and  the  floor  of  the  passage  is  thus  longer 
than  the  roof. 


Sound  and  Hearing 


203 


The  wall  of  the  outer  part  of  the  meatus  consists  of 
cartilage  which  is  continuous  with  that  of  the  auricle,  but 
round  the  deeper  part  of  the  tube  the  cartilage  is  absent, 
and  the  lining  of  skin  which  passes  inwards  from  without 
is  in  close  contact  with  the  bone  through  which  the  tube 


Fig.  ioi. — Diagram  of  the  ear  ;  natural  size.  1,  Auditory  nerve  ;  2,  internal  audi- 
tory  meatus  closed  by  the  cribriform  plate  of  bone  through  the  perforations  of 
which  the  branches  of  the  auditory  nerve  pass  to  the  ear  ;  3-8,  membranous 
labyrinth  composed  of  3,  utricle,  4,  semicircular  canals,  5,  saccule,  6,  duct 
of  the  cochlea  (the  coils  not  entirely  shown),  7,  endolymphatic  duct  with,  8,  its 
saccule  lying  inside  of  the  cranial  cavity ;  9,  lymphatic  space  surrounding 
the  membranous  labyrinth ;  10,  osseous  labyrinth  of  compact  bone  lying  in 
the  more  spongy  substance  of  the  petrous  bone,  11,  11  ;  12,  the  oval  window, 
filled  by  the  foot-plate  of  the  stirrup-bone ;  13,  the  round  window,  across 
which  is  stretched  the  internal  tympanic  membrane ;  14,  auricle  ;  15,  16, 
external  auditory  meatus  ;  15,  its  cartilaginous,  and,  16,  its  bony  part ;  17, 
tympanic  membrane  ;  18-20,  auditory  ossicles ;  18,  hammer ;  19,  anvil ;  20, 
stirrup ;  21,  middle  ear ;  22,  osseous,  and,  23,  cartilaginous  portion  of  the 
Eustachian  tube  ;  24,  cartilages  of  external  auditory  meatus.     (Schwalbe.) 

passes.  Towards  the  inner  part  of  the  meatus  the  skin 
is  very  thin,  and  this  is  especially  the  case  where  it  is  con- 
tinued as  an  epidermic  covering  over  the  fibrous  tympanic 
membrane.  At  the  outer  part  the  skin  is  thicker,  and  from 
it  spring  fine  hairs  slanting  outwards.      It  is  well  lubricated 


204  Physiology  of  the  Senses 

by  numerous  small  glands,  of  the  nature  of  sweat  glands 
much  modified,  which  secrete  a  waxy  substance  known  as 
cerumen.  This  material  has  a  brownish  colour  and  a  bitter 
taste.  The  form  of  the  canal  is  such  as  to  facilitate  the 
passage  outwards  of  the  wax,  but  sometimes  it  may  accumu- 
late in  such  quantity  as  to  diminish  the  power  of  hearing  to 
a  considerable  extent.  If  this  should  happen,  a  sharp  hard 
instrument  should  not  be  employed  for  its  removal,  as  much 
injury  might  thereby  be  inflicted  upon  the  tympanic  mem- 
brane. It  is  better  to  soften  the  wax  with  an  alkaline  or 
oily  fluid,  and  then  to  syringe  the  meatus  gently  to  remove 
the  debris.  The  outward -pointing  hairs  and  the  bitter 
adhesive  wax  form  together  a  valuable  guard  against  the 
entrance  of  foreign  bodies,  animate  or  inanimate,  into  the 
cavity  of  the  meatus,  a  provision  similar  to  what  we  find  in 
many  flowers  to  prevent  the  store  of  honey  from  being 
plundered  by  marauding  insects. 

3.  The  Middle  Ear. — The  middle  ear,  driwiy  or  tym- 
panum is,  in  the  adult,  about  an  inch  and  a  quarter  from 
the  free  surface,  and  is  thus  embedded  deeply  in  the  sub- 
stance of  the  temporal  bone.  Across  this  space  passes  the 
chain  of  bones  from  the  drum-head  to  the  internal  ear,  by 
means  of  which  the  movements  of  the  membrane  are  trans- 
mitted to  the  labyrinth  and  variations  of  pressure  effected. 
It  receives  air  at  atmospheric  pressure  through  the  Eus- 
tachian tube.  The  cavity  is  irregularly  wedge-shaped,  being 
wider  at  the  top  than  at  the  bottom,  and  is  larger  from 
before  backwards  than  from  side  to  side.  It  is  separated 
from  the  cranial  cavity  above  by  a  thin  layer  of  hard  bone, 
and  communicates  behind  with  a  set  of  spaces,  which  also 
contain  air,  lying  in  the  part  of  the  bone  which  can  be  felt 
as  a  prominence  behind  the  external  ear,  and  known  as  the 
mastoid  process.  The  outer  boundary  of  the  middle  ear  is 
largely  composed  of  the  tympanic  membrane,  although  it  is 


Sound  and  Hearing 


205 


to  be  noted  that  the  cavity  extends  upwards  into  the  bone 
above  the  membrane,  while  in  front  of  the  membrane  is  a 
fissure  in  the  bone,  known  as  the  fissure  of  G/aser,  from  its 
discoverer,  through  which  pass   a  nerve  (the  chorda  tym- 
fta?ii)  and  a  muscle  (the  laxator  tympani),  and  in  which, 
as  in  a  socket,  is  fixed  one  of  the  processes  by  which  the 
chain   of  bones   is   suspended.       The   membrane   itself  is 
firmly    fixed    in    a   groove, 
which  can  be  readily  seen 
in  a  macerated  bone  with 
the  naked  eye,  and,  though 
very   thin    and   semi-trans- 
parent,  it  consists  of  firm 
fibrous  tissue  lined  on  one 
side  by  skin,  on  the  other  by 
mucous  membrane.      Fig. 
102  represents  the  appear- 
ance of  the  tympanic  mem- 
brane of  the  left  ear  as  seen 
from  without,  and  Fig.  101 
shows    how   it   is    inclined 
obliquely  to  the  axis  of  the 
meatus,  both  transversely, 
and  from  above  downwards. 
The    fibres    of   the    mem- 
brane   consist   of  ordinary 
connective,     and     a     very 
small  amount  of  elastic,  tissue,  and  are  disposed  in  a  two- 
fold manner,  some  of  them   radiating   from   a  point,    the 
tunbo,  slightly  below  the  centre  of  the  membrane  to  the 
circumference,    while    others    are    arranged    concentrically 
around  the  same  point.    The  outer  surface  of  the  membrane  . 
is  covered  by  a  very  thin  layer  of  skin,  while  its  inner  tym- 
panic surface  is  lined  by  ciliated  epithelium.     The  first  of 


Fig.  102. — Left  tympanic  membrane  show- 
ing the  arrangement  of  its  fibres,  a 
anterior,  £  ..posterior  border ;  i,  flaccid 
part  of  the  membrane  ;  2,  short  process 
of  the  malleus ;  3,  umbo  of  the  mem- 
brane ;  between  2  and  3,  the  handle  of 
the  malleus  ;  4,  anterior  and,  5,  posterior 
end  of  the  tympanic  groove,  between 
which  are  seen  circular  fibres  attached 
to  the  short  process,  2.     (Schwalbe.) 


2o6 


Physiology  of  the  Senses 


Fig.    103.  —  Horizontal   section 


the  chain  of  bones  is  firmly  attached  to  the  fibrous  part  of 
the  membrane  in  such  a  way  that  the  central  part  of  the 
membrane  is  drawn  inwards  towards  the  tympanum,  form- 
ing the  umbo  (or  boss  of  a  shield), 
and  thus  the  disc  is  not  flat,  but 
slightly  conical,  and,  owing  to  the 
circular  fibres,  the  surface  towards 
the  meatus  is  convex.  This  cur- 
vature of  the  membrane,  though 
slight,  is  of  considerable  import- 
ance in  connection  with  the  re- 
sponse of  the  membrane  to  sonorous 
vibrations.  The  sound  waves  fall 
on    the    convex    surfaces   of   the 

through   the   labyrinth,   tym-  -.      .  ,,.  ,_,  ,  , 

Panum,andpartoftheexternal   radiating  fibres.     These  keep  the 
auditory  meatus  of  the  left  ear.   membrane  stretched  tightly,  except 

Between  d and  e,  the  tympanic       ,   ,-,        r  ,  , 

memlrane,   in  the  centre  of    at  the  fore  and   UPPer  Part>  wherc 

which  is  seen  the  handle  of  the   the  groove   of  attachment  is  de- 

malleus  cut  across  ;  e,  anterior     r    •       ,  j  .  v  i_  •    1 

„     f  .,      .  ,    ncient,  and  the  membrane  is  looser, 

wall    of    the    tympanum ;  j,  '  ' 

in  the  tympanum  above  the   thicker,  and  more  freely  supplied 

stapes  whose  base  is  inserted    ^  neryes  and  blood.vessels> 

into  the  fenestra  ovalis ;  q, 
the  stapedius  muscle  ;  h,  por- 
tion of  facial  nerve  ;  i,  tensor 
tympani  muscle  ;  k,  vestibular 
division,  and,  /,  cochlear  divi- 
sion of  the  auditory  nerve 
lying  in  the  internal  auditory 
meatus  ;  m,  cochlea  ;  n,  nerve  fenestra  OValtS,  OX  OVal  window,  is 

of  an  ovoid  or  kidney  shape,  and 
has  the  inner  end  of  the  ossicles 
of  the    ear    fastened 


The  inner  wall  of  the  tympanum, 
opposite  the  membrane,  is  irregular 
in  shape,  and  perforated  by  two 
apertures.   The  upper  of  these,  the 


going  to  ampull?e  of  semi- 
circular canals  ;  0,  section  of 
utricle  ;  p,  section  of  sac- 
cule ;  r,  section  of  semicircular 
canals.     (Riidinger.) 


into  it  by 
means  of  a  ligamentous  tissue. 
The  fenestra  ovalis  opens  from  the  middle  ear  into  the 
vestibule  of  the  labyrinth.  Lower  down  there  is  a  smaller 
and  more  rounded  aperture,  the  fenestra  rotunda,  or  round 
window,    leading   into    the    front    part    of   the    labyrinth, 


Sound  and  Hearing 


207 


known  as  the  cochlea,  but  closed  during  life  by  a  thin  mem- 
brane like  the  membrana  tympani — that  is  to  say,  com- 
posed of  fibrous  structure,  with  an  epithelial  lining  upon 
either  side,  and  having  a  slight  concavity  towards  the 
tympanum  (Fig.  101,  13). 

Between  and  in  front  of  the  above-mentioned  apertures 
is  a  rounded  elevation  called  the  firomo?itory,  which  corre- 
sponds to  the  first 
turn  of  the  cochlea 
(p.  228).  Behind  the 
oval  window  is  a 
very  small  process 
of  bone  perforated 
to  allow  the  passage 
of  a  minute  tendon, 
which  gives  attach- 
ment to  the  stapes 
(p.  211)  of  a  small 
muscle,  the  stape- 
dius, the  belly  of 
which  lies  in  a  space 
behind  the  tym- 
panum (Fig.  103,^). 

The      passage 
leading   away   from 


Fig.  104. — Incus  and  malleus  of  the  right  side  seen 
in  their  natural  position  in  the  tympanum,  i, 
Tympanic  membrane ;  2,  Eustachian  tube ;  3, 
tensor  tympani  muscle  seen  attached  to  the 
malleus ;  4,  anterior  ligament  of  the  malleus 
attached  to  the  processus  gracilis ;  5,  superior 
ligament  of  the  malleus  ;  6,  chorda  tympani  nerve  ; 
«,  b,  c,  sinuses  or  spaces  connected  with  the 
tympanum  in  which  the  ossicles  move  freely. 
(Schwalbe.) 


the  front  of  the  tym- 
panum is  divided  into  two  parts  by  a  little  ledge  of  bone, 
known  as  the  processus  cochleariformis,  the  upper  part  con- 
taining the  fleshy  part  of  a  muscle,  the  tensor  ty?npani, 
whose  tendon  crosses  the  tympanum  to  be  inserted  into 
the  malleus,  the  lower  going  forwards  as  the  Eustachian 
tube  (Fig.  103,  i). 

The  Eustachian  Tube. — The  mucous  membrane  of  the 
Eustachian   tube    is    continuous    behind   with   that   of  the 


208  Physiology  of  the  Senses 

tympanum,  in  front  with  the  pharynx  or  upper  part  of  the 
throat.  When,  under  certain  conditions,  this  mucous  mem- 
brane becomes  swollen,  the  lumen  of  the  tube  may  be 
blocked,  and  air  does  not  pass  readily  to  and  fro  between 
the  throat  and  the  middle  ear.  Then  the  pressures  upon 
opposite  sides  of  the  membrane  becoming  different,  the 
membrane  is  too  much  stretched,  does  not  respond  so  well 
as  usual  to  sonorous  vibrations,  and  one  becomes  slightly 
deaf.  It  is  commonly  held  that  the  Eustachian  tube  is 
open  only  during  swallowing,  and  the  positive  and  negative 
experiments  of  Valsalva  are  brought  forward  in  proof  of 
this.  The  positive  experiment  is  performed  as  follows  : 
Close  the  mouth  and  nostrils,  and  then,  while  making  the 
movements  of  a  forced  expiration,  swallow.  The  air  in  the 
pharynx  is  at  more  than  atmospheric  pressure,  but  does 
not  force  its  way  into  the  tympanum  until  the  tube  is 
opened  during  swallowing.  Then  the  condensed  air  pene- 
trates into  the  middle  ear,  raises  the  pressure  there,  and 
the  drum-head  is  forced  slightly  outwards  and  made  more 
taut.  The  tightening  of  the  membrane  gives  rise  to  a  peculiar 
sensation  referred  to  the  region  of  the  ears,  and  similar  to 
what  is  sometimes  felt  after  yawning. 

We  may  directly  observe  this  movement  by  inspection 
of  the  membrane  during  the  act.  The  principle  of  the 
negative  experiment  is  much  the  same.  Instead,  however, 
of  making  a  forced  expiration,  we  close  the  mouth  and 
nostrils,  raise  the  chest  as  in  forced  inspiration,  and  swallow. 
The  air  in  the  throat  being  at  less  than  atmospheric  press- 
ure, when  the  Eustachian  tube  is  opened  the  pressure  in 
the  middle  ear  is  reduced,  and  the  tympanic  membrane 
moves  inwards  by  the  atmospheric  pressure  in  the  meatus. 
We  have  also  met  with  a  gentleman  who  had  the  voluntary 
control  of  the  tube,  so  that  he  could  open  or  close  it  at 
pleasure.      The  advantage  of  having  the  tube  closed  at  all 


Sound  and  Hearing 


209 


times,  except  when  we  swallow,  lies  in  this,  that  were  it 
always  open  there  would  be  too  much  reverberation  caused 
in  our  ears  by  the  sound  of  our  own  voice.  This,  however, 
cannot  affect  the  ears  during  swallowing,  because  then  the 
lower  part  of  the  pharynx  is  cut  off  from  the  openings  to 
the  nose  and  ears  by  the  meeting  of  opposite  muscles,  and 
the  lifting  of  the  uvula  and  soft  palate.  From  all  this  it 
follows  that  one,  and  probably  the  most 
important,  function  of  the  Eustachian 
tube  is  to  equalise  atmospheric  pressure 
on  the  two  sides  of  the  drum-head. 

The  Chain  of  Bones. — Across  the 
cavity  of  the  tympanum  stretches  the 
chain  of  little  bones  or  ossicles  (Fig. 
104),  to  which  frequent  reference  has 
already  been  made.  This  corresponds 
to  the  single  bone  in  the  frog's  ear,  which 
stretches  from  the  tympanic  membrane 
to  the  entrance  to  the  inner  ear,  but,  as 
we  shall  see,  the  chain  confers  consider- 
able mechanical  advantage.  It  consists 
from  without  inwards  of  the  malleus  or 
hammer  bone,  the  incus  or  anvil  bone, 
and  the  stapes  or  stirrup  bone. 

The  body  or  head  of  the  malleits 
(Fig.  105)  is  situated  above  the  level  of 
the  tympanic  membrane,  and  it  gives  off 
downwards  a  comparatively  strong  process,  the  handle  of 
the  hammer,  which  is  firmly  affixed  to  the  fibrous  layer  of 
the  membrane.  And  just  as  a  flattened  beam  will  bear 
a  greater  downward  pressure  when  placed  edgewise  than 
when  laid  flat,  so  the  handle  of  the  malleus,  being  flattened, 
is  placed  edgewise  towards  the  tympanic  membrane,  thus 
combining  lightness  with  power.     Another  process,  the  ftro- 

P 


Fig.  105. — The  malleus 
or  hammer  bone  seen 
from  in  front.  1,  The 
head  ;  2,  the  processus 
gracilis  foreshortened ; 

3,  the  short  process  ; 

4,  the  manubrium  in- 
serted into  the  tym- 
panic membrane.  The 
surface  of  the  joint 
with  the  incus  is  not 
seen,  as  it  faces  back- 
wards.    (Schwalbe.) 


210 


Physiology  of  the  Senses 


cessus  gracilis  more  slender  and  elongated  than  the  handle, 
passes  forwards  from  the  junction  of  the  head  with  the 
handle,  and  is  firmly  fixed  by  ligaments  to  the  little  fissure 
in  the  bone  in  front  of  the  tympanic  membrane.  This  pro- 
cess is  of  interest  as  constituting  one  end  of  the  axis  upon 
which  the  chain  of  bones  rotates.  The  head  of  the  malleus 
is  rounded,  and  attached  to  the  roof  of  the  tympanum  by  a 
small  ligament.  It  bears  upon  its  pos- 
terior aspect  a  smooth  surface  for  arti- 
culation with  the  incus.  The  head  is 
connected  with  the  handle  by  a  con- 
stricted neck,  immediately  below  which 
we  find,  on  the  inner  side  of  the  handle, 
the  point  of  attachment  of  the  tensor 
tympani  muscle,  and  on  the  outer  part 
Right  incus  a  sma^  bony  prominence  whicha  im- 
pinging upon  the  tympanic  membrane, 
causes  a  projection  outwards  of  the 
tion  that  locks  with    membrane  at  that  point.      The  laxator 

malleus  to  prevent  over  .  ,  ,        ,  ,      r 

movement ;   4,    short   tympani  muscle  passes  backwards  from 
process   for   posterior   the  fissure  of  Glaser,  to  be  attached  to 

attachment      of      the     .-■  ,       c   ,-,  ,-,  ,  ,-i 

,       .        „.  „.  the  neck  of  the  malleus,  lust  above  the 

bone ;    5,   elliptic  area  '  •> 

on  median  side  of  short   origin  of  the  processus  gracilis. 

process ;    6,  long    pro-  The    .  Qr  anvil.shaped  bone  (Fig. 

cess    ending    in    lenti-  '  r  v      ° 

cuiar knob ;  7,  entrance  106),  lies  behind  the  malleus,  and  is 
jointed  to  it  by  a  saddle-shaped  surface. 
A  short  process,  pointing  backwards,  and 
fixed  to  the  posterior  wall  of  the  tympanum  by  ligaments, 
forms  the  posterior  end  of  the  axis  of  rotation  of  the  chain  of 
bones.  A  longer  process,  corresponding  to  the  conical  pro- 
jection of  an  anvil,  points  almost  vertically  downwards,  but, 
at  its  lower  extremity,  bends  inwards  and  ends  as  a  little 
flattened  knob,  the  lenticular  process,  which  in  early  life  is 
a   separate  bone,  the  lenticular  bone.     A  small  eminence, 


Fig.    106. 

or  anvil  bone,  X  4.  1, 
Body  ;  2,  joint  surface 
for  malleus  ;  3,  projec- 


of      nutrient       blood- 
vessel.    (Schwalbe.) 


Sound  and  Hearing  211 

immediately  below  the  surface  of  articulation  with  the 
malleus,  should  be  noted,  as  it  fits  into  a  corresponding 
depression  in  the  malleus  and  prevents  undue  rotation. 

The  stapes,  or  stirrup-shaped  bone  (Fig.  107),  is  fixed  in 
a  horizontal  plane,  and  at  right  angles  to  the  descending 
process  of  the  incus.  The  head  of  the  stirrup  is  jointed  to 
the  lenticular  process  of  the  incus.  Inwards  from  the  head 
is  a  slight  constriction,  the  neck,  and  from  this  arise  the 
two  arms  of  the  stirrup.  These  are  fixed  at  their  inner 
end  into  an  oval -shaped  plate  of  bone,  the  base  of  the 
stirrup,    which    again    fits    into    the    oval  4, 

window.       The    stirrup    could    move    out-  &SL... .$ 

wards  and  inwards  freely  but  for  the  firm      g"fj^^^\ 
short  fibres    which   unite    its    base   to   the     2— if  1—3 

margins  of  the   aperture.     The  space  be-        Iv^^qI 
tween  the  arms  is  filled  during  life  by  a  „ 

_  Fig.   107. — Stapes  or 

thin  membrane,  the  arms  being  grooved  stirrup  bone  seen 
to  receive  it.      By  this  arrangement,  light-      from  above>  x  4-  i, 

Base ;     2    anterior, 

ness  and  strength  are  secured  in  the  same  3  posterior  limb; 
way  as  we  make  wheels  with  spokes  instead      4>  head ;  s>  neck  > 

_,.,,.  _.  .  r     .  6,  groove  into  which 

Of  SOlld    dlSCS.       The    tendon  Of   the  Stape-        membrane  is  fixed 

dius  muscle  is  attached  to  the  back  part  of     which  fills  the  open- 

ing.     (Schwalbe.) 

the  neck  of  the  stapes. 

Movements  of  the  Bones. — The  malleus  and  incus 
rotate  almost  as  one  bone  on  a  horizontal  axis,  passing  fore 
and  aft  between  the  attachments  of  the  slender  process  of 
the  malleus  in  front,  and  the  short  process  of  the  incus 
behind.  The  plane  of  rotation  is  consequently  at  right 
angles  to  that  of  the  tympanic  membrane,  or  across  the 
cavity  of  the  tympanum.  When,  then,  the  handle  of  the 
malleus  is  pushed  inwards  towards  the  mesial  plane  of  the 
head,  the  head  of  the  malleus  moves  outwards,  carrying 
with  it  the  body  of  the  incus,  any  excess  of  movement  being 
prevented  by  the  suspensory  ligament  of  the  malleus.      The 


212 


Physiology  of  the  Senses 


body  of  the  incus  rotating  outwards,  its  descending  process 
moves  inwards  synchronously  with,  and  parallel  to,  the 
handle  of  the  malleus,  and  the  tip  of  the  process  is  thus 
moved  inwards  and  slightly  upwards,  and  pushes  the  base 
of  the  stapes  into  the  fenestra  ovalis.  There  is  also  a  slight 
rotation  of  the  stapes  in  a  vertical  plane,  and  the  upper 
border  of  the  base  of  the  stapes  has  a  somewhat  greater 
movement  than  the  under  side.  We  see,  then,  that  when, 
by  compression  of  the  air  in  the  external  meatus,  the  tym- 
panic membrane  is  forced  inwards,  the  base  of  the  stapes 
will  also  be  forced  inwards,  and  the  pressure  on  the  internal 
ear  will  be  increased. 

Again,  when  the  air  of  the  external  meatus  is  rarefied, 
and  the  pressure  on  the  inner  side  of  the  membrane  becomes 
greater  than  on  the  outside,  the  membrane  is  forced  out- 
wards, carrying  with  it  the  handle  of  the  malleus.  Then 
the  head  of  the  malleus  above  the  axis  rotates  inwards, 
carrying  with  it  the  body  of  the  incus,  and  the  long  process 
of  the  incus,  moving  away  from  the  mesial  plane,  carries  the 
stapes  with  it,  and  pressure  on  the  internal  ear  is  diminished. 
The  distance  through  which  the  base  of  the  stapes  can 
move  is  very  small,  and  hence  it  might  happen  that  a  very 
loud  sound,  causing  the  tympanic  membrane  to  vibrate 
through  a  comparatively  large  distance,  might  tear  the 
stapes  from  its  attachments.  This,  however,  is  guarded 
against  in  several  ways.  In  the  first  place,  a  somewhat 
dense  ligament  passes  from  the  upper  part  of  the  external 
wall  of  the  tympanum  to  the  head  of  the  malleus,  and  this 
receives  the  impact  of  the  head  of  the  malleus  as  upon  an 
elastic  cushion,  and  may,  when  the  head  of  the  malleus 
tends  to  move  too  far  inwards,  restrain  it  from  moving  too 
freely.  Secondly,  the  process  below  the  upper  joint  of  the 
incus  fits  into  a  depression  in  the  malleus,  and  when  the 
handle  of  the  malleus  tends  to  move  too  far  inwards,  this 


Sound  and  Hearing  213 

projection  locks  into  the  opposing  socket  like  the  tooth  of  a 
cog-wheel,  and  prevents  too  great  movement  inwards.  On 
the  other  hand,  if  the  handle  of  the  malleus  rotates  outwards 
excessively  the  tooth  is  withdrawn,  and  the  saddle-shaped 
joint  coming  into  play,  the  lower  part  of  the  joint  tends  to 
gape,  and  the  incus  does  not  move  so  far  outwards  as  the 
malleus. 

Further,  the  chain  of  bones  acts  like  a  bent  lever,  the 
arm  of  the  incus  being  only  two-thirds  of  the  length  of  the 
malleus.     When  the  lower  end  of  the  handle  of  the  malleus, 
fixed  in  the  umbo  of  the  tympanic  membrane,  moves  through 
a  given  distance,   the    stapes   fixed  to  the 
lower  end  of  the  process  of  the  incus  will 
only  move  through  two-thirds  of  this  dis- 
tance.     But  while  the  excursion  distance 
is  diminished,  we  know  from  the  principle 
of  the    lever  that   the   force  with   which  it   FlG- 108.— Diagram 
moves  must  be  increased  by  one-half.    There      itverage^ctiontf 
is  thus  diminished  amplitude  of  movement,      the   malleus  and 
but  increase   of  power.      This  is  a  distinct      ££££££ 
advantage,  considering  the  small  power  that      process    of    the 
sound  waves  have  of  moving  the  tympanic      incus* 
membrane,  and  the  firmness  with  which  the  base  of  the  stapes 
is  fixed.      This  increase  of  power  is  augmented  by  the  fact 
that  the  tympanic  membrane  has  roughly  an  area  twenty 
times  as  great  as  the  base  of  the  stapes.      Thus  the  tym- 
panic membrane  concentrates  its  power  upon  an  area  only 
one-twentieth  of  its  size,  and  this,  increased  by  the  shorter 
arm  of  the  lever  (of  the  incus),  must  give  a  force  at  least 
thirty  times  as  great  as  that  with  which  the  handle  of  the 
malleus  is  moved  at  the  umbo  of  the  tympanum.      Another 
reason  why  the  stapes  cannot  move  far  is  found  in  the  firm- 
ness  of  the   fibres   of  the  membrana  tympani,  and   of  its 
attachment  to  the  handle  of  the  malleus  ;  extensive  move- 


214  Physiology  of  the  Senses 

ment  of  the  membrane  is  thus  prevented.  Lastly,  where 
the  membrane  might  move  too  freely,  we  have  the  action  of 
the  tensor  tympani  muscle  coming  into  play.  By  the  pull 
inwards  of  this  muscle  upon  the  handle  of  the  malleus,  the 
tension  of  the  membrane  is  increased,  and  its  extent  of 
vibration  correspondingly  diminished.  But  this  brings  us 
to  a  consideration  of  the  manner  in  which  membranes 
respond  to  sonorous  vibrations. 

Response  of  the  Tympanic  Membrane  to  Sound  "Waves. 
— The  physical  cause  of  the  sensation  of  sound  is  the  rapid 
vibration  to  and  fro  of  the  molecules  of  an  elastic  medium 
when  these  have  been  set  in  motion  by  a  sudden  shock. 
The  particles,  when  disturbed,  vibrate  to  and  fro  till  they 
regain  their  former  equilibrium.  Such  vibration  may  be 
transmitted  from  •  molecule  to  molecule  through  solids, 
liquids,  or  gases.  Thus  the  arm  of  a  tuning-fork,  when 
set  in  vibration,  causes  an  alternate  condensation  and  rare- 
faction of  the  air  in  the  space  through  which  it  moves. 
With  each  successive  to  and  fro  movement  of  the  fork 
another  alternation  of  change  of  density  is  set  up,  and  this 
is  propagated  outwards  in  all  directions  from  the  fork  as  a 
centre.  The  direction  of  movement  of  the  particles  in  a 
sound  wave  is  not  transverse  to  the  direction  in  which  the 
wave  is  moving,  but  in  the  same  direction.  Hence  they 
are  said  to  be  longitudinal  waves,  as  distinguished  from  the 
transverse  movements  characteristic  of  waves  of  light,  or  of 
waves  moving  on  the  surface  of  water.  Such  longitudinal 
waves  can  readily  be  set  up  in  solids,  as,  for  example,  in  a 
wooden  rod  by  friction,  and  on  account  of  the  closeness  to 
one  another  of  the  molecules  in  solids  such  vibrations  are 
transmitted  with  great  rapidity.  But  rods,  strings,  or  mem- 
branes may  be  caused  to  vibrate  transversely  to  their  length 
or  plane,  as  when  a  violin  string  is  pulled  aside  by  the  bow, 
or   a   drum    is   beaten.       If   these   vibrations   be  in    quick 


Sound  and  Hearing 


215 


succession,  they  will  give  rise  to  sound  waves  in  air.  In 
this  case  it  will  be  noticed  that  while  the  particles  of  the 
solid  body  are  moving  transversely  to  the  length  of  the  rod 
or  string,  or  the  plane  of  the  membrane,  their  direction  of 
vibration  is  still  longitudinal  in  so  far  as  the  direction  of 
the  transmission  of  sound  is  concerned. 

The  impulses  given  to  the  air  by  a  vibrating  string  are  of 
a  complex  type,  for  while  it  may  vibrate  as  a  whole,  and  give 
forth  a  series  of  waves,  which  combin- 
ing  excite  the  sensation  of  a  sound  or 
tone,  this  fundamental  tone  is  always 
modified  by  the  presence  of  overtones 
produced  by  the  simultaneous  vibra- 
tion of  segments  of  the  string  (Fig.  109). 
In  the  case  of  a  rod  or  string  these  seg- 
ments are  respectively  a  half,  a  third, 
a  fourth,  and  so  on,  of  the  length  of 
the  whole  rod  or  string,  and  the  num- 
bers of  vibrations  given  forth  by  these 
segments  are  respectively  twice,  three 
times,  four  times,  and  so  on,  that  of 
the  fundamental  tone.  In  the  case  of 
plates  or  membranes,  the  number  and 
character  of  the  overtones  are  more 
difficult  to  determine,  being  dependent 
on  the  form  and  elasticity  of  the 
plates,  the  manner  in  which  they  are 

set  vibrating,  and  the  number  of  vibrations.  The  smaller 
and  the  more  tightly  stretched  a  membrane  is,  the  faster 
will  be  its  rate  of  vibration  and  the  higher  the  pitch  of  the 
sound  thereby  caused.  On  a  large  vibrating  membrane  the 
surface  is,  as  it  were,  subdivided  into  many  portions  of  vary- 
ing sizes,  some  small,  some  large,  each  vibrating  at  a  rate 
peculiar  to  itself,  and  thus  giving  rise  to  a  complicated  set 


A 


B 


Fig.  109. — Diagram  of 
string  vibrating  so  as  to 
give  forth  its  fundamental 
tone  (A),  and  its  first 
upper  partial  tone  or 
octave  (B). 


216  Physiology  of  the  Senses 

of  aerial  vibrations.  Conversely,  if  the  air  is  vibrating  at 
any  of  the  rates  at  which  the  membrane,  or  parts  of  it,  may 
vibrate,  the  membrane  will  begin  to  vibrate  in  response. 
Suppose  two  violin  strings,  or  two  tuning-forks,  are  tuned 
to  the  same  pitch  and  placed  close  to  one  another ;  if  one 
of  these  be  set  vibrating  the  other  will  also  begin  to  vibrate 
at  the  same  rate  ;  but  strings  or  rods  will  not  respond  so 
readily  as  membranes  to  a  variety  of  tones.  Membranes 
respond  more  readily  to  aerial  vibrations  than  plates  do, 
because  of  the  smaller  mass  of  matter  requiring  to  be 
moved,  and  the  consequently  greater  flexibility  of  the 
surface.  A  drum-head  will  move  freely  to  and  fro  under  a 
blow  which  will  cause  almost  no  apparent  result  upon  a 
thick  plate. 

The  application  of  these  facts  to  the  action  of  the  tympanic 
membrane  in  hearing  is  not  far  to  seek.  In  the  first  place, 
the  membrane  is  small,  very  thin,  its  fibres  are  inelastic, 
and  it  is  firmly  but  not  evenly  stretched  in  all  its  parts. 
From  its  thinness  it  can  respond  to  aerial  impulses  of  very 
faint  kinetic  energy. 

This  receptivity  we  have  seen  may  be  interfered  with  by 
the  accumulation  of  hardened  wax  upon  the  membrane. 

The  peculiar  arrangement  of  the  fibres  of  the  membrane 
makes  it  respond  to  sounds  of  widely-varying  pitch.  The 
fibres  radiating  from  the  umbo  to  their  varying  points  of 
attachment  in  the  tympanic  groove  constitute,  as  it  were,  a 
vast  number  of  strings  of  varying  lengths,  each  of  which 
will  respond  most  readily  to  its  own  particular  tone.  Again, 
the  concentric  circular  fibres  may  be  regarded  as  surround- 
ing a  series  of  nearly  circular  discs  of  gradually  increasing 
size,  and  therefore  of  different  vibratile  capacities.  Further, 
von  Helmholtz  has  shown  that  the  shallow  conical  form  of 
the  membrane,  the  slight  outward  convexity  of  its  fibres, 
renders  it  less  liable  to  have  a  fundamental  tone  only,  and 


Sound  mid  Hearing  217 

increases  its  receptivity  for  all  varieties  of  sounds.  It  has 
been  found  that  if  a  handle  be  attached  to  a  flat  disc,  and 
the  disc  be  then  curved  like  the  tympanic  membrane,  it 
ceases  to  have  a  fundamental  tone.  This  property  of  the 
drum-head  is  of  paramount  importance  in  hearing,  as  it 
leaves  the  ear  free  from  the  disadvantage  of  having  all  tones 
but  one  overburdened  by  a  preponderating  fundamental 
tone.  Almost  every  ear  will  respond  to  tones  having  as 
low  a  frequency  as  30  vibrations  per  second,  while  certain 
acute  ears  may  hear  tones  caused  by  40,000  vibrations  per 
second. 

The  receptivity  of  the  tympanic  membrane  for  sounds  of 
high  pitch,  that  is  to  say,  sounds  due  to  a  large  number  of 
vibrations  per  second,  is  enhanced  by  the  action  of  the  tensor 
tympani  muscle  (Fig.  104).  When  this  muscle  contracts  it 
pulls  the  handle  of  the  malleus,  and  with  it  the  tympanic 
membrane,  inwards,  and  thus  tightens  the  membrane  just  as 
a  drum-head  is  made  more  tense  when  it  is  braced  up. 
The  fibres  being  tighter,  their  play  is  diminished,  and  they 
respond  more  readily  to  vibrations  following  in  quick 
succession.  On  the  other  hand,  by  the  action  of  the 
laxator  tympani,  the  membrane  becomes  more  flaccid  and 
responds  better  to  sounds  of  low  pitch. 

It  has  been  suggested  that  the  power  which  many  trained 
musicians  have  of  recognising  the  absolute  pitch  of  a  note 
may  depend  to  some  extent  upon  the  sense  of  muscular  effort 
arising  from  varying  degrees  of  contraction  of  the  tensor 
tympani.  In  such  cases  long  practice  in  the  determination 
of  the  pitch  of  notes  gives  rise  to  such  delicacy  of  judgment 
that  there  seems  to  be  an  intuitive  and  direct  recognition  of 
pitch,  and  not  only  may  the  pitch  of  a  sound  heard  by  the 
musician  be  named  by  him,  but  he  may  sing  a  note  of  any 
given  pitch  that  he  desires  without  the  aid  of  tuning-fork 
or  instrument.      For  the  performance  of  this  latter  act,  it  is 


2i8  Physiology  of  the  Senses 

not  unlikely  that  the  parts  unconsciously  assume  the  neces- 
sary degree  of  tension  before  the  sound  is  uttered,  just  as 
we  are  apt  to  make  involuntary  contortions  of  the  facial 
and  other  muscles  when  performing  complicated  or  difficult 
actions. 

One  important  factor  in  the  regulation  of  the  tympanic 
membrane  has  still  to  be  mentioned.  If  we  strike  the  keys 
of  a  piano  and  hold  them  down  so  as  to  prevent  the 
dampers  touching  the  strings,  the  vibration  of  the  strings 
will  go  on  for  a  considerable  time  ;  but  when  we  release 
the  keys,  and  the  dampers  touch  the  strings,  the  vibration 
stops.  In  the  ear  the  handle  of  the  malleus  attached  to 
the  tympanic  membrane  acts  as  a  damper.  If  the  mem- 
brane went  on  vibrating  after  the  sound  wave  had  ceased, 
there  might  be  interference  with  other  succeeding  sounds, 
but  the  duration  of  the  vibration  is  cut  short  by  the  resist- 
ance offered  by  the  chain  of  bones.  The  development  of 
overtones  in  the  membrane  is  likewise  prevented,  and  the 
ear  is  rendered  more  acute  in  the  discrimination  of  different 
sounds  following  one  another  in  rapid  succession,  and  each 
tone  is  heard  pure,  and  not  interfered  with  by  those  which 
have  immediately  preceded  it.  There  is  a  further  provision 
in  the  structure  of  the  internal  ear  for  differentiation  of  sounds, 
but  this  we  will  refer  to  afterwards. 

Transmission  of  Vibration  by  the  Auditory  Ossicles. — 
We  have  next  to  consider  how  auditory  vibrations  are 
conveyed  to  the  internal  ear.  It  has  been  experimentally 
determined  that  sound  is  mainly  transmitted  through  the 
middle  ear  by  the  movement,  as  a  whole,  of  the  chain  of 
bones.  No  doubt  where  these  are  absent,  or  have  been 
rendered  immovable  by  disease,  a  person  may  still  be  able  to 
hear,  but  the  acuteness  of  hearing  will  be  largely  interfered 
with.  As  to  the  nature  of  the  movement  of  the  bones 
there  is  a  common  consensus  of  opinion.     It  will  be  readily 


Sound  and  Hearing-  219 

understood  that  the  movement  of  a  solid  body  may  be  the 
resultant  of  many  constituent  elements.  The  earth  rotates 
upon  its  axis  whilst  it  moves  round  the  sun.  In  a  red-hot 
cannon  ball  projected  through  the  air,  the  molecules  of  the 
metal  are  in  a  state  of  extremely  rapid  movement  with 
reference  to  each  other,  as  well  as  in  transmission  through 
space.  In  a  tense  string  set  into  transverse  vibration  there 
must  be  a  continual  lengthening  and  shortening  of  the 
string,  or  in  other  words,  a  change  in  position  of  the  mole- 
cules relatively  to  one  another  and  in  the  direction  of  the 
length  of  the  string  as  well  as  the  transverse  movement  of 
the  string  as  a  whole.  The  longitudinal  movement  of  the 
particles  is  invisible,  the  transverse  movement  is  visible,  to 
the  naked  eye.  The  former  we  call  molecular,  the  latter 
molar  movement.  Probably  there  is  some  molecular  move- 
ment of  the  ossicles  of  the  ear,  but  the  presence  of  joints 
must  largely  interfere  with  this,  and  the  movement  is  mainly 
of  the  bones  as  a  whole,  that  is  to  say,  a  molar  move- 
ment, a  movement  that  may  be  seen  with  the  eye.  While 
this  is  so,  we  must  be  careful  to  distinguish  between  the 
amount  of  movement  of  the  bones  and  the  length  of  the 
sound  wave.  The  length  of  a  sound  wave  is  dependent  not 
upon  the  amplitude  of  movement  of  the  sounding  body — 
that  determines  the  intensity  or  loudness  of  the  sound — 
but  upon  the  number  of  vibrations  made  in  a  given  time 
by  the  sounding  body.  In  Fig.  no,  p.  220,  A  represents 
a  long  wave  of  small  amplitude  of  movement,  B  short 
waves  with  greater  amplitude.  The  length  of  the  wave  is 
measured  by  the  interval  between  two  successive  points  in 
like  phase  relatively  to  one  another.  Thus  in  A,  we  must 
move  from  a  to  c  in  order  to  get  two  particles  in  like 
condition  of  velocity  and  direction  of  movement,  so  we  say 
that  ac  is  the  length  of  the  wave.  Now  the  distance 
through  which  a  sound,  wave  will  pass  in  any  medium  in  a 


220  Physiology  of  the  Senses 

given  time  depends  upon  the  elasticity  and  density  of  the 
body  in  question.  Through  air,  sound  waves  pass,  on  an 
average,  at  the  rate  of  1120  feet  per  second.  If,  then,  a 
body  makes  a  complete  to  and  fro  vibration  only  once  each 
second,  the  first  movement  must  have  passed  1020  feet 
before  the  second  begins,  or  in  other  words,  the  wave- 
length is  1 1 20  feet.  If  the  body  performs  a  complete 
vibration  twice  in  a  second  the  distance  between  two 
points  of  like  condensation  and  rarefaction  will  only  be  one- 
half  of  1 120  feet,  or  560  feet.  The  more  rapid  the  rate 
of  vibration,  the  faster  will  wave  succeed  wave,  and  the 
shorter  will  the  wave  be.  The  ear  can  readily  distinguish 
as  a  musical  tone  sounds  due  to  vibrations  following  each 


1  .* 


II  a 


Fig.  iio. — Diagram  illustrating  (I.)  long  waves  of  small  amplitude,  and  (II.) 
short  waves  of  greater  proportional  amplitude. 

other  thirty  times  in  a  second.  The  wave-length  in  such  a 
case  would  be  1120  —  30  =  37  feet  approximately,  while 
certain  ears  can  hear  a  sound  due  to  40,000  vibrations  per 
second,  in  which  case  the  wave-length  will  be  1120  feet  -i- 
40,000,  or  approximately  \  of  an  inch.  But  in  either 
case  it  will  be  seen  that  the  bones  of  the  ear  cannot  move 
through  the  "  length  of  the  wave,"  but  rather  that  the  time 
of  recurrence  of  like  condition  of  condensation  or  rare- 
faction at  the  drum-head  gives  rise  to  our  appreciation  of 
differences  of  pitch.  Regularly  succeeding  stimuli  going  to 
the  auditory  nerve  at  the  rate  of  say  thirty  times  a  second 
will  give  rise  to  a  sensation  of  a  sound  of  low  pitch,  and  if 
at  the  rate  of  say  4000,  to  a  sensation  of  a  sound  of  high 
pitch.      The  length  of  the  wave  is  of  importance  in  regulat- 


Sound  and  Hearing  221 


<^> 


ing  the  number  of  times  per  second  the  drum-head  will 
vibrate,  taking  into  account  the  rate  of  the  transmission  of 
sound  waves  through  air ;  but  the  breadth  of  the  ear,  and 
even  of  the  whole  head,  may  only  form  a  very  small  part  of 
the  length  of  the  wave.  A  tuning-fork  bowed  gently  will 
give  a  sound  of  the  same  pitch  as  the  same  fork  bowed 
strongly.  In  the  one  case  we  cannot  see  any  movement  in 
the  limbs  of  the  fork  ;  in  the  latter  the  sharp  outline  of  the 
limbs  is  lost,  and  we  can  see  at  once  that  the  limbs  are  in 
motion.  Similarly  in  the  ear.  With  weak  sounds  the 
drum-head  hardly  moves,  and  the  ossicles  seem  to  be  at 
rest,  but  if  the  sound  is  loud,  the  drum-head  and  the  bones 
may  be  seen  in  motion.1  With  very  loud  sounds,  when 
many  molecules  of  air  have  been  suddenly  compressed  into 
a  small  space,  the  pressure  upon,  and  consequent  move- 
ment of,  the  tympanic  membrane  is  very  great,  and  the 
force  may  even  be  so  excessive  as  to  cause  rupture  of  the 
membrane,  just  as  windows  are  sometimes  shattered  by  a 
violent  and  consequently  loud  explosion. 

While  in  ordinary  circumstances  the  tympanic  mem- 
brane is  usually  thrown  into  vibration  through  the  medium 
of  the  air  in  the  external  meatus,  it  should  be  borne  in 
mind  that  it  may  be  set  in  motion  also  by  transmission 
of  vibrations  through  the  bones  of  the  skull. 

If  a  tuning-fork  is  struck,  and  its  handle  pressed  against 
the  teeth,  a  molecular  movement  is  transmitted  to  the 
membrane  with  such  energy  as  to  set  the  membrane  and 
ossicles  into  visible  molar  movement.      We  can  illustrate 

1  A  preparation  can  be  made  of  the  ear  of  a  dead  cat.  The  middle 
ear  is  laid  open  by  removing  a  small  portion  of  its  wall.  After  lightly 
dusting  the  interior  with  lycopodium  powder,  it  is  strongly  illuminated 
and  examined  with  a  microscope  of  moderate  power.  When  the 
vibrations  of  an  organ  pipe,  sounding  loudly,  are  directed  into  the 
external  ear,  little  brilliant  specks  of  lycopodium  powder  may  be  seen 
to  vibrate. 


222  Physiology  of  the  Senses 

this  by  placing  a  number  of  marbles  in  a  row,  and  touching 
one  another.  If  a  smart  tap  be  given  to  the  marble  at  one 
end  of  the  row,  it  will  not  apparently  move,  nor  will  the 
intervening  members  of  the  series,  but  the  last  marble  of 
the  row  will  fly  off  as  if  directly  struck.  The  energy  of  the 
blow  is,  in  this  case,  transmitted  through  the  molecules  of 
the  marbles,  and  is  sufficient  to  give  rise  to  visible  move- 
ment in  the  last  member  of  the  series.  So  the  movement 
transmitted  through  the  bones  of  the  skull  gives  rise  to  free 
movement  of  the  tympanic  membrane,  and  through  it  to 
the  internal  ear.  Trial,  however,  will  show  that  the  tym- 
panic membrane  responds  better  to  the  vibrations  of  the 
air  in  the  meatus  than  to  those  transmitted  through  the 
head.  If  a  tuning-fork  be  struck,  and  its  handle  held 
between  the  teeth  till  the  sound  has  apparently  ceased,  and 
if  then  the  fork  be  held  opposite  the  ear,  the  sound  will  be 
distinctly  heard  again.  We  may  attribute  this  to  the 
greater  mobility  of  the  molecules  of  air  in  the  meatus  than 
that  of  the  molecules  of  the  bones  of  the  head.  They 
move  more  freely  to  and  fro,  and  under  a  feebler  stimulus, 
than  the  molecules  of  the  bones,  and  thus  the  membrane 
responds  more  readily  to  the  tuning-fork  held  to  the  ear. 
Still,  although  both  membranes  be  absent,  the  ear  is  quite 
capable  of  hearing  and  of  distinguishing  musical  sounds  by 
the  direct  stimulation  of  the  internal  ear,  and  its  apprecia- 
tion of  pitch  cannot  be  affected,  inasmuch  as  this  is  due  to 
the  physical  fact  of  a  recurrence  of  stimuli  at  definite 
intervals  of  time.  The  intensity  of  the  sound  will,  how- 
ever, be  diminished,  because,  as  we  have  seen,  the  arrange- 
ment of  membrane  and  ossicles  gives  a  mechanical  advan- 
tage in  the  way  of  increased  power. 


Sound  and  Hearing 


223 


The  Internal  Ear 

We  have  already  said  that  the  internal  ear  consists  of  a 
closed  sac  formed  by  an  invagination  of  part  of  the  skin  at 
a  very  early  period  of  life,  and  that  the  nerve* of  hearing 
ends  in  this  sac.  We  have  now  to  consider  the  form  of 
the  internal  ear,  the  mode 
of  ending  of  the  auditory 
nerve  in  it,  and  the  manner 
in  which  its  structure  is 
adapted  to  the  function  of 
hearing.  And,  in  the  first 
place,  let  it  be  noted  that 
modern  research  tends  to 
confirm  a  conjecture  made 
long  ago  that  the  front  part 
of  the'  internal  ear,  the 
cochlea,  has  to  perform  an 
entirely  different  function 
from  the  posterior  part.  In 
correspondence  with  this, 
the  auditory  nerve  has  been 
shown  to  consist  of  two 
nerves  (Fig.  103,^,  /)  which, 
arising  in  different  parts  of 
the  brain,  are  united  by 
connective  tissue  in  the 
greater  part  of  their  course, 


Fig.  hi.— Right  bony  labyrinth  viewed 
from  the  outside  (x  2  J,  and  natural 
size).  The  more  spongy  material  of  the 
petrous  bone  has  been  separated  from 
the  dense  bony  wall  of  the  labyrinth. 
1,  The  vestibule  ;  2,  fenestra  ovalis  or 
oval  window ;  3,  superior  semicircular 
canal  ;  4,  horizontal  or  external  semi- 
circular canal ;  5,  posterior  semicircular 
canal ;  *  *  *  ampullae  or  dilatations  of 
semicircular  canals  ;  6,  first  coil  of  the 
cochlea  ;  7,  second  coil ;  8,  apex ;  9, 
fenestra  rotunda  or  round  window. 
(Sommerring.) 


but  separate  again  as  they 

approach  their  termination,  and  end  in  organs  which  differ 

widely  in  appearance  from  each  other. 

The  posterior  portion  of   the   sac    is    contained   in    the 
bony  cavity  known  as  the  vestibule  and  semicircular  ca?ials. 


224 


Physiology  of  the  Senses 


We  may  imagine  the  canals  as  having  been  cut  off  from 
the  main  body  of  the  sac  by  the  meeting  and  agglutination 
of  opposite  parts  of  the  original  cavity,  just  as  if,  were  we  to 
press  together  between  thumb  and  finger  the  opposite  sides 
of  a  bag  near  one  of  its  corners,  we  would  form  a  canal  or 
passage  communicating  at  each  end  with  the  main  cavity  of 
the  bag  (Fig.  1 1 2).  This  main  cavity  in  the  ear  is  known  as 
the  utricle  (Fig.  113);  it  is  oblong  in  shape,  being  about  one- 
fourth  of  an  inch  long,  and  communicates  behind  and  above 
with  three  semicircular  canals  (Fig.  in,  3,  4,  5)  which  lie 
respectively  in  three  planes,  one  horizontal  and  two  vertical, 
and  all  exactly  at  right   angles   to   each   other   like   three 


Fig.  112. — Diagrammatic  representation  of  the  manner  in  which  the  semicircular 
canals  are  formed  from  a  primary  cavity.     (See  text.) 

adjacent  sides  of  a  cube.  From  the  direction  in  which  the 
curves  are  inclined,  the  canals  are  named  respectively  the 
horizontal  or  external,  the  antero- posterior,  or  simply  the 
posterior  and  the  transverse  or  superior  canals.  Each 
canal  has  one  of  its  openings  into  the  utricle  dilated  to 
form  what  is  known  as  an  ampulla  (Fig.  1 1 1),  the  other 
end  passing  into  the  utricle  without  enlargement,  and  the 
undilated  ends  of  the  canals  in  the  vertical  planes  unite 
with  one  another  before  passing  to  the  utricle,  so  that  there 
are  only  five  openings  for  the  canals  into  the  utricle,  three 
of  which  are  provided  with  ampullae. 

The  utricle  lies   in  the  vestibule.      Below,  and  in  close 
apposition    to,  the   utricle,    and,  like  it,   contained   in   the 


Sound  and  Hearing 


225 


vestibule,  we  have  the  saccule  (Fig.  113),  a  smaller  and 
more  rounded  space  than  the  utricle.  These  two  cavities  are 
formed  by  a  constriction  of  the  primary  vesicle,  and  even 
in  adult  life  are  in  connection  with  each  other  by  a  long 
narrow  tube  of  a  Y  shape,  the  ductus  endolymfihaticus 
(Fig.  113),  one  part  of  which  actually  penetrates  through 
the  bone  into  the  cavity  of  the  skull,  and  lies  enclosed  by 
the  membranes  surrounding  the  brain.  The  saccule,  by  a 
narrow  tube,  the  canalis  reuniens  (Fig.  113),  communicates 
with  the  long  finger-like  projection,  the  canal  of  the  cochlea, 
which  is  packed  away  in 
small  space  by  being  wound 
two  and  a  half  times  round  a 
central  supporting  pillar  of 
bone,  the  modiolus  (Fig.  1 17). 
The  auditory  nerve,  enter- 
ing the  bone  containing  the 
internal    ear    by    a    passage 

Called  the  i?lter?ial  auditory  Fig.  113.  —  Membranous  labyrinth 
meatus    divides     as    it    enters         (diagrammatic),     c,  Cochlea;  s,  sac- 

meaius,  divides,  as  it  enters       culeunitedby/)  the  ductus  endolym- 

the   bony    labyrinth,  into   two        phaticus,  with  u,  the  utricle,  arising 

j-    •    •  ,  from  which  are  seen  the  three  semi- 

mam  divisions,  one  going  to        .     .  , 

0         °  circular  canals. 

the  cochlea,  and  the  other  to 

the  vestibular  part  of  the  membranous  labyrinth,  the  latter 
branch  quickly  dividing  further  so  as  to  supply  a  terminal 
branch  to  the  utricle,  the  saccule,  and  the  ampullae  of  the 
semicircular  canals,  and  to  these  parts  alone. 

The  membranotis  labyrinth  has  for  its  outer  coating  a 
layer  of  connective  tissue  from  which  numerous  processes 
pass  to  the  fibrous  lining  of  the  bone.  The  spaces 
between  the  processes,  similar  to  other  lymph  spaces 
throughout  the  body,  are  lined  with  flat  cells  and  filled  with 
a  somewhat  viscous  fluid.  The  connective  tissue  is  homo- 
logous with  the  true  skin,  and  like  it  contains  blood-vessels. 

Q 


226 


Physiology  of  the  Senses 


The  inner  lining  of  the  sac,  except  where  the  nerves   end, 
consists  of  a  single  layer  of  flattened  cells.      In  one  portion 


o     v  —  £ 

«J  (4  5 

"■'  '"  y  s  ,- 

K  t~  ,n  ja  ^ 

n     H  u  «  (S 

u   g  S  c 


of  the  utricle  and  of  the  saccule  lies  a  small  oval  spot,  or 
macula,  and  in  the  ampulla  of  each  canal  a  ridge  or  crista 


Sound  and  Hearing 


227 


Fig.  115. — Epithelial 
cells  from  macula 
acustica  of  the 
utricle. 


which,  since  they  contain  the  termination  of  the  vestibular 
nerves,  are  known  respectively  as  a  macula  or  crista  acustica. 
Over  these  the  epithelium  is  stratified,  being  mainly  made 
up  of  thread-like  columnar  cells  (Fig.  115),  having  a  well- 
marked  nucleus,  and  supporting  another 
set  of  nucleated  cylindrical  cells,  whose  free 
surfaces  bear  bunches  of  stiff  rod-like  hairs 
which  are  often  adherent  one  to  another, 
and  are  known  as  the  auditory  hairs.  Some 
observers  have  described  the  hairs  as  pass- 
ing through  a  membrane  similar  to  that 
found  in  the  cochlea  (p.  236)  ;  but  this  has 
been  disputed.  The  terminal  twigs  of  the 
auditory  nerve,  passing  through  the  con- 
nective tissue  which  forms  the  main  sub- 
stance of  the  prominence  or  ridge,  lose 
their  outer  sheaths  and  pass  as  naked  axis- 
cylinders  into  the  epithelium,  where  their  mode  of  termina- 
tion is  not  definitely  known.  Some  suppose  that  they  end 
in  the  cells,  others  that  they  simply  surround  them  with  a 
nest  of  fine  fibrils  ;  but,  from  analogy  with  the  other  sense 
organs,  we  may  conjecture  that  they  are  at 
least  stimulated  by  the  agitation  of  the 
hair- cells.  The  free  ends  of  the  auditory 
hairs  are  embedded  in  a  soft  mucous  mate- 
rial, the  cuptcla,  in  which  are  often  found 
small  crystals  consisting  largely  of  carbonate 

from    the    cupula  J  °         °     J 

above  the  human   of  lime,  called  otoconia, or  otoliths  (Fig.  1 16). 
macula  acustica.     The  function  0f  this  covering  is  unknown, 

though  it  has  been  supposed  to  act  as  a  damper  to  the 
vibration  of  the  auditory  hairs.  It  may  possibly  be  driven 
mechanically  against  the  points  of  the  hairs  by  vibrations  of 
sound,  and  thus  increase  the  sensitiveness  of  the  hairs  to 
such  vibrations. 


o 


Fig.  116. — Otoconia 


228 


Physiology  of  the  Senses 


The  Cochlea. — We  come  now  to  consider  the  struc- 
ture of  the  cochlea  (Gr.  coc/ilias,  a  snail  with  spiral  shell), 
which  is  a  tubular  cavity  coiled  in  a  spiral  manner  round 
a  central  pillar  called  the  modiolus.  The  part  of  the  mem- 
branous labyrinth  which  it  contains  is  much  smaller  in 
cross  section  than  the  bony  space,  and  is  known  as  the 
ca?ialis  cochlearis.  It  is  fixed  in  the  whole  of  its  course, 
except  at  its  closed  end,  to  either  side  of  the  cochlea,  having 
a  broad  surface  of  attachment  on  the  outside,  but  a  very 
narrow  one  towards  the  median  column.      Indeed,  we  find 

here  that  the  cochlear  canal 
is  only  attached  on  its  inner 
aspect  to  the  free  edge  of 
a  shelf  wnich  winds  round 
the  central  pillar,  projects 
outwards  into  the  lumen  of 
the  cochlea,  and  is  known 

Fig.   n7.-The  osseous  cochlea  divided  aS  the  lamina  &™Hs  OSSea, 

through  the    middle,  X  5.      1,   Central  or  Spiral  plate  of  bone.        It 

canal  of  the  modiolus  in  which  lies  the  ^      consists    of   a    double 

cochlear   nerve ;    2,   the  spiral   osseous  J 

lamina;    3,    scala    tympani ;    4,    scala  plate    of  bone,  between   the 

vestibuli  ;   5,  spongy  bone  of  modiolus  surfaces  of  which  the  nerveS 
near  the  spiral  canal,  8.     (Arnold.) 

pass  out  from  the  central 
column  to  enter  the  cochlear  canal.  In  a  section  made 
transversely  through  one  of  the  whorls  of  the  cochlea, 
we  see  then  three  spaces  represented  in  Fig.  118.  The 
upper  space,  containing  perilymph,  is  in  connection,  at 
its  beginning,  with  the  vestibule,  and,  as  it  winds  round 
towards  the  apex  of  the  cochlea,  it  is  known  as  the  stair- 
way from  the  vestibule  or  scala  vestibuli.  At  the  summit 
of  the  cone  it  bends  round  the  closed  end  of  the  cochlear 
canal  and  the  free  hook-like  end  or  hamulus  of  the  lamina 
spiralis,  by  a  little  passage  called  the  helicotrema,  and 
communicates    with    a    descending    space   which,   winding 


Sound  and  Hearing  229 

round  the  modiolus,  ends  at  the  fenestra  rotunda,  whose 
membrane  closes  the  opening  into  the  middle  ear.  This 
lower  space  is  known  as  the  scala  tympani.  The  two 
scalae  are  lined  with  a  connective  tissue  membrane  which 
is  thickened  on  the  outer  wall  to  form  the  spiral  ligament, 
first  described  by  Bowman,  and  the  free  surface  of  the 
membrane  is  covered  with  a  single  layer  of  flattened  cells. 
The  scalas  being  in  connection  with  each  other  at  the  top 


Fig.  118. — Section  through  one  of  the  coils  of  the  cochlea  (diagrammatic).  SV, 
Scala  vestibuli ;  ST,  scala  tympani ;  CC,  canal  of  the  cochlea ;  Iso,  lamina 
spiralis  ossea,  or  spiral  plate  of  bone  ;  Us,  limbus  of  the  spiral  lamina ;  R, 
Reissner's  membrane ;  ss,  spiral  sulcus  or  groove ;  t,  tectorial  membrane  ; 
CO,  organ  of  Corti ;  b,  basilar  membrane  ;  Isp,  spiral  ligament  ;  nc,  cochlear 
nerve  ;  gs,  spiral  ganglion  in  course  of  cochlear  nerve.     (After  Henle.) 

of  the  whorl,  and  being  filled  with  perilymph,  the  pressure 
of  the  fluid  in  the  two  spaces  must  be  the  same  when  the 
ear  is  at  rest.  If,  by  the  movement  of  the  stapes,  the 
pressure  of  the  fluid  in  the  vestibule  be  increased  or 
diminished,  there  must  be  a  corresponding  change  of 
pressure  transmitted  from  the  scala  vestibuli  to  the  scala 
tympani,  and  this  may  be  effected  either  directly  through 
the  cochlear  canal  or  through  the  helicotrema.  The  fluids 
of  the  ear  being  practically  incompressible  there  must  be  a 


230  Physiology  of  the  Senses 

corresponding  movement  of  the  membrane  closing  the 
fenestra  rotunda. 

Upon  the  upper  surface  of  the  spiral  bony  shelf,  and 
near  its  free  border,  is  a  thickening  of  the  connective  tissue 
known  as  the  limbics.  This  thins  away  as  it  covers  the 
free  edge  of  the  shelf,  and  a  groove  is  formed — the  sulcus 
spiralis  (Fig.  118) — whose  free  borders  are  known  respec- 
tively as  the  vestibular  and  tympanic  lips. 

The  Cochlear  Canal. — In  cross  section,  the  canal  of  the 
cochlea  is  roughly  triangular  in  shape,  the  apex  being 
attached  to  the  spiral  plate  of  bone,  the  base  to  the  outer 
wall  of  the  cochlea.  That  part  of  the  wall  of  the  canal 
which  looks  towards  the  scala  vestibuli  arises  from  the  upper 
surface  of  the  spiral  shelf  a  little  nearer  the  modiolus  than 
the  limbus,  and  stretches  as  a  thin  fibrous  membrane, 
known  as  Reissnefs  membrane,  to  the  outer  wall.  It  is 
lined  on  its  vestibular  side  by  flattened  cells,  while  the 
internal  surface  is  clothed  with  more  cubical  cells,  some  of 
which  have  probably  a  secretory  function. 

The  wall  of  the  cochlear  canal,  which  takes  part  in  the 
formation  of  the  scala  tympani,  stretches  from  the  tympanic 
lip  of  the  spiral  lamina  to  the  spiral  ligament,  and  is  known 
as  the  lamina  spiralis  membranacea,  or  basilar  membrane. 
It  is  indistinctly  fibrous  towards  its  inner  attachment,  but 
in  its  outer  two-thirds  shows  a  radial  fibrillation  as  of  rod- 
like fibres  embedded  in  a  homogeneous  matrix.  This  part 
of  the  structure  is,  as  we  shall  see,  probably  of  considerable 
importance  in  the  appreciation  of  the  pitch  of  sounds. 

The  tympanic  surface  is  lined  with  cells,  often  of  a 
spindle  shape,  which  lie  transversely  to  the  fibres  above 
them,  and,  at  one  part  immediately  below  the  organ  of 
Corti  about  to  be  described,  we  find  a  small  blood-vessel, 
the  vas  spirale,  which  ensures  a  good  blood  supply  to  the 
superjacent  structures. 


Sound  and  Hearing 


231 


The  Organ  of  Corti. — The  epithelium  upon  the  upper,  or, 
with  reference  to  its  position  in  the  head,  anterior  surface  of 


Fig.  119. — Cross  section  of  the  human  cochlear  duct  at  the  junction  of  the  first 
and  second  turns  of  the  cochlea,  X  100.  1,  Outer  wall  (part  of  the  spiral 
ligament)  reaching  from  b  to  c ;  2,  vestibular  wall,  or  Reissner's  membrane, 
from  a  to  c  ;  tympanic  wall  from  atob;  3,  lamina  of  bone  ;  4,  its  vestibular 
lip ;  5,  its  tympanic  lip  ;  6,  nerves  of  hearing  passing  to  epithelium  at  7  ;  8, 
internal  spiral  groove  with  flattened  epithelium ;  9,  basilar  membrane ;  10, 
its  tympanic  covering  ;  11,  basilar  crest  of  spiral  ligament ;  12,  prominence 
of  spiral  ligament  with  blood-vessel ;  between  11  and  12,  the  external  spiral 
groove  ;  13,  vascular  layer  ;  14,  spiral  papilla  (epithelium  of  Corti's  organ) ; 
near  14,  the  outer  hair-cells  and  Deiter's  cells ;  further  inwards  the  rods  of 
Corti  covering  the  tunnel ;  internal  to  this  the  inner  row  of  hair-cells ;  15,  the 
tectorial  membrane.     (After  Retzius.) 

the  basilar  membrane  is  of  a  highly  specialised  type,  and 
more  especially  that  part  which  rests  upon  the  inner  half  of 
the  membrane.    This  part  is  commonly  known  as  the  organ 


232 


Physiology  of  the  Senses 


of  Corti,  from  the  Italian  Marquis  of  that  name  who  first 
gave  a  detailed  description  of  it.  When  we  examine  sections 
made  transversely  to  the  length  of  the  canal,  we  find  a 
peculiar  structure  resting  upon  the  basilar  membrane 
immediately  adjoining  its  inner  line  of  attachment.  This 
consists  of  a  set  of  elongated  rod-like  cells  arranged  in  two 
rows  throughout  almost  the  whole  length  of  the  cochlear 
canal,  and  known  as  the  outer  and  inner  rods  of  Corti. 
These  rod-cells,  rising  from  the  membrane,  meet  at  their 
upper  ends  like  the  beams  of  a  sloping  roof,  and,  together 

with  the  membrane,  enclose  a 
space  called  the  tunnel.  The 
individual  rods  have  a  cylin- 
drical form  and  an  expanded 
base,  by  which  they  are  fixed 
to  the  basilar  membrane.  The 
-r,  T  ,      .        ,     ,   upper   ends    of   the   rods    are 

r  ig.   120.— Inner  and  outer  rods  of       rr 

Corti  from  the  cochlea  of  a  guinea-    enlarged,  but  flattened  at  the 

pig,  X  275.     A,  Inner  rod-cell ;  B,     ^        where    h  .     ^^ 

outer  rod-cell.     In  both  are  seen —  7  J 

i,  the  foot  piece;  2,  the  body ;  and,    with   adjoining   rods,   and  the 

3,  upper  end  of  rods  ;   4    nucleus    inner    heads    haye  thdr 

and  protoplasm,     (bchwalbe.)  r 

outer  aspect  a  socket  into 
which  fit  the  rounded  heads  of  the  outer  row  of  rods. 
From  the  head  of  each  rod  there  projects  outwards  a 
flattened  process,  those  of  the  inner  row  overlapping 
those  of  the  outer.  The  inner  rods  are  about  a  half  more 
numerous  than  the  outer,  so  that  two  outer  rods  fit  into 
three  of  the  inner  row.  At  the  base  of  each  rod  we 
find  a  nucleus  and  granular  protoplasmic  material,  while 
the  main  substance  of  the  rod  exhibits  no  structure,  or 
merely  a  faint  longitudinal  striation.  The  rods  being 
placed  in  line,  and  all  the  head-plates  being  similar  in 
size  and  appearance,  they  present,  when  seen  from  above, 
a   remarkable   resemblance   to   the  key-board   of  a  piano. 


Sound  and  Hearing 


233 


Fibres  of  the  auditory  nerve  pass  between  the  rods  and 
across  the  tunnel,  which,  during  life,  contains  also  a  colour- 
less jelly-like  intercellular  substance  (Fig.  121). 


Fig.  121. — Surface  view  of  the  spiral  papilla  of  Corti's  organ  from  the  topmost  coil 
of  a  rabbit's  cochlea,  from  the  inner  hair-cells  to  the  cells  of  Deiter.  (After 
Retzius.)  Highly  magnified,  i,  Inner  row  of  hair-cells  ;  2,  boundary  line  of 
their  surface  ;  3,  cuticle  of  the  inner  hair-cells,  each  showing  eight  hairs  ;  to  the 
left  an  extra  inner  cell  is  present ;  4,  flattened  tops  of  the  inner  rods  of 
Corti ;  5,  outer  border  of  these  plates ;  these  completely  cover  the  tops  of 
the  outer  row  of  rods,  seen  between  6  and  7  ;  at  6  is  seen  the  inner  border 
line  of  attachment  of  the  heads  of  the  outer  rods.  From  the  tops  of  the  outer 
rods  are  seen  at  7  the  processes  to  the  phalangse,  narrow  at  8,  and  widening 
at  9  to  form  part  of  the  lamina  reticularis.  10,  Phalangse  of  the  first  row. 
ii,  Phalange  of  the  second  row.  10-12  are  the  cuticular  end  plates  of  the 
three  rows  of  Deiter's  cells.  In  the  interspaces  between  these  appear  three 
rows  of  outer  hair-cells,  each  showing  eight  hairs,  arranged  in  horse-shoe 
shape,  projecting  from  their  free  cuticular  surface. 

The  Inner  Hair-Cells. — Just  to  the  inner  side  of  the 
rods  of  Corti  we  find  a  row  of  columnar  cells  whose  free 


234 


Physiology  of  the  Senses 


surface  is  on  a  level  with  the  head  of  the  inner  rods  upon 
which  they  rest.     Each  of  these  columnar  cells  has  project- 


FiG.  122. — Radial  section  through  the  tympanic  wall  of  the  middle  of  the  cochlear 
duct  of  the  guinea-pig,  X  212.  1  and  2,  Upper  and  lower  plates  of  the 
osseous  spiral  lamina ;  3,  spiral  ganglion  ;  4,  spiral  bundle  of  medullated 
nerve  fibres  ;  5,  medullated  nerve  fibres  radiating  outwards  between  the  bony 
plates  of  the  spiral  lamina  ;  6,  thin  connective  tissue  lining  bone  (periosteum)  ; 
7,  limbus  of  the  spiral  lamina ;  8,  its  vestibular  lip ;  9,  its  tympanic  lip, 
through  which  at  10  the  nerve  fibres,  losing  their  medullary  sheath,  pass  to 
the  epithelium  ;  11,  beginning  of  Reissner's  membrane  ;  12,  union  of  tympanic 
lip  with  basilar  membrane ;  13,  nucleated  transparent  layer  of  the  basilar 
membrane  ;  14,  layer  of  basilar  fibres ;  15,  cellular  lining  of  basilar  mem- 
brane ;  16,  epithelium  of  internal  spiral  groove ;  17,  inner  supporting  cells, 
below  which  the  nerves  emerge;  18,  inner  hair-cells  ;  19,  inner  rod  of  Corti, 
a,  nucleus  and  protoplasm  ;  20,  outer  rod  of  Corti  with,  b,  its  nucleus  and  pro- 
toplasm ;  c,  cross  section  of  spiral  bundle  of  nerve  fibres  winding  up  with  the 
tunnel ;  from  it  the  nerve  fibres,  d,  pass  outwards  between  the  outer  rods  of 
Corti  to  the  outer  hair-cells  ;  21,  outer  hair-cells  in  three  rows  alternating 
with  phalangar  processes,  22,  of  Deiter's  cells,  23  ;  24,  supporting  fibres 
of  Deiter's  cells  ;  25,  cells  of  Hensen  ;  26,  cells  of  Claudius  ;  27,  membrana 
tectoria  ;  28,  its  marginal  thickening.     (Schwalbe.) 

ing  from  its  free  surface  from  fifteen  to  twenty  short  stiff 
hairs  arranged  in  a  crescentic  line,  whose  convexity  faces 
outwards.      The  attached  ends  of  the  hair-cells  are  conical 


..-13 


Sound  and  Hearing  235 

in  shape,  and  do  not  come  down  to  the  basilar  membrane, 
but  are  connected  with,  or  closely  invested  by,  terminal 
fibrils  of  the  auditory  nerve.  There  may  also  be  seen 
around  and  below  the  lower  ends  of  the  hair-cells  a  number 
of  nuclei.  These  belong  to  elongated  filamentous  cells, 
which,  arising  from  the  beginning  of  the  basilar  membrane, 
pass  to  the  surface  between,  and  to  the  inside  of,  the  hair- 
cells,  and,  in  all  probability,  act  like  the  rods  of  Corti  as 
supporting  structures.  From  the  inner  row  of  hair-cells 
epithelial  cells,  at  first  columnar,  then  more  cubical  or  even 
flattened,  line  the  spiral  groove  already  referred  to,  but  the 
overhanging  part  of  the  vestibular  lip  of  the  limbus  is  devoid 
of  epithelium,  and  is  broken  up  by  slight  radial  markings 
into  a  set  of  projections  known  as  the  auditory  teeth. 

Outer  Hair-Cells. — To  the  outer  side  of  the  rods  of 
Corti  we  find  rows  of  hair-cells  and  supporting  cells  similar 
in  many  ways  to  the  row  found  to  the  inside  of  the  rods. 
In  the  human  ear  there  are  usually  four  rows  of  hair-cells, 
but  there  may  be  only  three,  or  as  many  as  five,  rows  in 
certain  parts  of  the  canal.  In  the  ears  of  lower  mammals 
there  are  seldom  so  many  rows  as  in  man. 

The  hair-cells  of  the  outer  row  are  likewise  columnar, 
have  short  stiff  hairs  arranged  in  a  semicircular  or  horse- 
shoe shape — convexity  outwards — on  their  free  surface,  a 
nucleus  surrounded  by  granular  protoplasm,  and  nearer 
their  free  border  a  dark  pigmented  spot  known  as  He?iserCs 
spot.  The  lower  ends  of  the  hair-cells  do  not  pass  down  to 
the  basilar  membrane,  but,  like  the  inner  row  of  hair-cells, 
are  in  contact  with  the  terminal  fibrils  of  the  auditory  nerve. 
Closely  apposed  to  the  outside  of  each  of  the  hair-cells  in 
the  outer  rows  is  a  supporting  structure,  known  as  Better's 
cell  (see  Fig.  122),  which,  arising  by  a  thicker  nucleated 
part  from  the  basilar  membrane,  gradually  becomes  nar- 
rower and  passes,  as  a  small  cylindrical  process,  to  the  free 


236  Physiology  of  the  Senses 

surface.  Here  the  Deiterian  cells  are  fixed  to  fiddle-shaped 
plates — phalanges — which,  uniting  with  adjoining  plates, 
and  with  the  processes  from  the  heads  of  the  rods  of  Corti, 
form  a  fenestrated  or  reticiclated  membrane,  in  the  meshes 
of  which  lie  the  free  ends  of  the  hair-cells.  Each  hair-cell 
is  thus  fixed  to  and  supported  by  a  structure,  which  is  itself 
inserted  at  either  end  into  a  membrane,  and  thus  the  com- 
ponent cells  are  firmly  held  in  their  respective  places,  and 
we  can  see  that  any  movement  of  the  basilar  membrane 
must  be  at  once  communicated  to  the  hair-cells  through  the 
medium  of  Deiter's  cells. 

Outside  of  the  rows  of  hair-cells  we  find,  for  a  short  dis- 
tance, a  row  of  columnar  cells,  devoid  of  hairs,  and  having 
no  direct  connection  with  the  auditory  nerve.  They  are 
known  as  Hensen's  cells,  and  they  soon  merge  into  a  layer 
of  cubical  cells,  the  cells  of  Claudius,  which  cover  the  outer 
third  of  the  basilar  membrane,  and  are  continued  over  the 
spiral  ligament  and  that  part  of  the  cochlear  canal  which 
is  in  contact  with  the  outer  cochlear  wall. 

The  spiral  ligament  into  which  the  basilar  membrane  is 
fixed,  consists  in  the  main  of  connective  tissue,  but  spindle- 
shaped  cells  have  been  described  as  existing  in  it,  which,  as 
first  suggested  by  Bowman,  are  supposed  to  be  muscular, 
and  whose  function  would  be  to  tighten  the  basilar  mem- 
brane, and  adapt  it  for  variations  of  pitch.  The  spiral 
ligament  is  vascular,  and  at  one  part  a  slight  elevation  {vas 
pro7ninens)  is  made  by  a  vein  (Fig.  1 19). 

It  will  be  seen  that  the  neuro-epithelium  of  the  cochlea 
resembles,  in  many  respects,  that  found  in  the  vestibular 
part  of  the  internal  ear.  This  likeness  is  further  increased 
by  the  fact  that  we  find,  lying  in  the  cochlear  canal,  fixed 
at  one  end  to  the  vestibular  lip  of  the  limbus,  and  at  the 
other  free  or   attached   to  the  outer  part  of  the  organ  of 


Sound  and  Hearing  237 

Corti,  a  thickish  layer  of  fibrous  tissue  known  as  the  mem- 
brana  tectoria.  This  may,  as  conjectured  in  the  case  of 
the  cupula,  act  as  a  damper  when  resting  on  the  hair-cells, 
but  its  action  is  not  known. 

Innervation  of  the  Cochlea.— The  cochlea  is  supplied 
by  a  branch  of  the  auditory  nerve.  The  modiolus  or  cen- 
tral column,  round  which  the  cochlea  is  coiled,  is  hollowed 
out  in  a  conical  fashion,  the  space  being  filled  by  the  coch- 
lear nerve,  which,  comparatively  thick  at  first,  soon  lessens 
in  diameter  by  giving  off  numerous  branches  which  pass 
out  into  the  bony  spiral  shelf.  Before  reaching  their  ulti- 
mate destination,  however,  the  fibres  pass  into  a  mass  of 
ganglionic  nerve-cells  of  a  spindle  or  bi-polar  form,  which 
form  a  continuous  spiral  from  the  base  to  nearly  the  apex 
of  the  cochlea,  known  as  the  spiral  ganglion  (Fig.  122). 
From  this  the  fibres  emerge  in  bundles  which  coalesce  to 
form  finer  bundles.  These  passing  radially  outwards,  be- 
tween the  opposing  surfaces  of  the  spiral  lamina,  emerge  in 
little  furrows  or  canals  at  the  tympanic  lip,  called  foramina 
nervina,  and,  losing  here  their  primitive  sheath  and  white 
medullary  substance,  pass  as  bare  axis-cylinders  into  the 
neuro-epithelium  of  Corti's  organ. 

The  nerve  fibres  do  not  seem  to  pass  directly  after 
emerging  from  the  bony  plate  to  the  hair-cells  opposite. 
They  seem  rather  to  bend  round  and  run  in  the  direction 
of  the  cochlear  spiral,  some  below  the  inner  row  of  hair- 
cells,  some,  after  entering  the  tunnel,  through  interstices 
between  the  rods  of  Corti,  and  some  in  spaces  between  each 
row  of  the  Deiter's  cells  supporting  the  outer  row  of  hair- 
cells.  There  are  thus  an  inner  spiral  strand,  a  spiral  strand 
of  the  tunnel,  and  three  or  four  outer  spiral  strands.  From 
these  spirals  are  given  off  the  ultimate  fibrils  which  proceed 
to  the  hair-cells.  Whether  they  pass  into  these,  or  simply 
into  contact  with  them,  is  not  definitely  known.      We  may, 


238  Physiology  of  the  Senses 

however,  feel  assured,  both  from  analogy  and  from  careful 
study  of  the  structure,  that  the  hair-cells  are  the  true  ter- 
minal organs  of  the  auditory  nerve,  that  they  alone  can 
respond  to  auditory  vibrations,  and  set  up  sensory  impulses 
in  the  auditory  nerve,  and  that  the  other  cells  of  Corti's 
organ  are  merely  accessory  in  function.  In  birds,  for 
instance,  the  cochlea  is  very  rudimentary,  consisting  of  a 
small  protuberance  from  the  saccule,  and  containing  only 
hair-cells  on  a  basilar  membrane  and  no  rods  of  Corti.  It 
may  seem  strange  that  in  birds,  even  in  the  sweetest  song- 
sters, the  part  of  the  ear  which  seems  specially  devoted  to 
the  appreciation  of  musical  tones  should  be  ill  developed  ; 
but  it  must  be  remembered  that  the  quality  and  variety  of 
tones  of  the  bird's  song  are  vastly  inferior  to  those  of  the 
human  voice,  nor  has  the  brain  of  the  bird  the  development 
necessary  for  the  due  recognition  of  the  variety  of  spunds 
which  the  human  brain  can  differentiate.  In  the  human 
ear  itself,  the  structure  of  Corti's  organ  varies  as  we  pass 
from  the  beginning  to  the  end  of  the  canal.  At  first,  where 
it  unites  with  the  canalis  reuniens  (p.  225),  it  is  lined  with 
ordinary  epithelium.  Then  the  organ  of  Corti  has  at  first 
only  three  rows  of  hair-cells  ;  farther  on,  four  rows  appear, 
and  in  some  ears  five.  At  the  closed  end  of  the  canal,  the 
neuro-epithelium  is  again  awanting,  and  gives  place  to  a 
simple  squamous  epithelium. 

Observations  are  still  required  with  regard  to  the  com- 
parative powers  of  ears  as  regards  the  appreciation  of  vary- 
ing sounds  according  to  the  number  of  hair-cells  which  may 
be  present.  While  the  general  principle  of  formation  of 
Corti's  organ  remains  the  same  throughout  the  whole  length 
of  the  cochlea,  the  grouping  of  the  supporting  cells,  and 
more  especially  those  of  Hensen,  gives  different  appearances 
at  different  levels  of  the  spiral.  It  is  also  noteworthy  that 
the  basilar  membrane  varies  in  breadth,  not,  as  was  at  one 


Sound  and  Hearing  239 

time  supposed,  narrowing  from  base  to  apex,  but  actually  in- 
creasing from  .2 1  mm.  (y^g  inch)  to  .36  mm.  (nearly  T|T  inch) 
(Retzius)  in  breadth  as  it  ascends.  Thus,  if  we  regard  its 
radial  fibres  as  corresponding  to  the  strings  of  a  musical 
instrument,  such  as  the  harp,  those  fibres  which  lie  at  the 
base  of  the  cochlea,  and  consequently  nearest  the  vestibule, 
would  compare  with  the  short  strings  of  the  harp,  which 
vibrate  rapidly,  and  give  forth  sounds  of  high  pitch,  while 
those  at  the  apex  of  the  cochlea  correspond  to  the  long 
strings  which  emit  a  bass  note.  If,  as  has  been  supposed, 
this  analogy  is  not  a  merely  fanciful  one,  it  is  manifest  that 
we  have  in  this  arrangement  the  greatest  mechanical  advan- 
tage, tones  of  short  wave-length  obtaining  immediate 
response,  while  those  of  greater  wave-length   must  travel 


i€ 


£j    -< 35VWI. ^cv, 

Fig.  123. — Diagram  illustrating  change  in  breadth  of  the  basilar  membrane  from 
base  to  apex  of  cochlea  ;  the  length  of  the  diagram  is  about  twice,  the  breadth 
about  ten  times,  the  actual  dimensions  ;  the  numbers  in  the  diagram  indicate 
in  millimetres  the  size  of  the  structure  in  the  ear,  not  the  lengths  of  the  lines. 

farther.  The  basilar  membrane  being,  according  to  Retzius, 
about  35  mm.  (i-|th  inch)  in  length,  the  accompanying 
diagram  (Fig.  123)  represents  on  an  enlarged  scale  the  com- 
parative breadth  of  the  membrane  in  different  parts '  in 
relationship  to  each  other,  and  to  the  length  of  the  canal. 
The  actual  difference  in  the  length  of  the  fibres  is,  as  will  be 
seen,  very  little,  and  it  should  further  be  noticed  that  the 
distinct  fibrillation  of  the  membrane  is  well  marked  only  in 
the  outer  side  of  the  membrane,  between  the  outer  rows 
of  hair-cells  and  the  attachment  of  the  membrane  to  the 
spiral  ligament.  If  this  part  alone  be  considered,  we  find 
that  the  ratio  is  somewhat  altered — namely,  from  .075  mm. 
at  the  base  to  .  1 26  mm.  at  the  apex,  or  nearly  1  :  2  instead  of 
3.5.      The  difference  in  absolute  size  may  seem  very  little, 


240  Physiology  of  the  Senses 

but  we  must  always  bear  in  mind  the  exceeding  minuteness 
of  all  the  parts  involved,  and  the  extreme  delicacy  with 
which  so  small  an  organ  must  be  constructed  in  order  to 
give  such  complex  and  varied  results  as  does  the  human 
ear.  The  presence  of  what  seem  to  be  contractile  cells 
in  the  spiral  ligament  lends  colour  to  the  supposition  that, 
in  the  length  and  tension  of  the  fibres  of  the  basilar  mem- 
brane, we  are  to  look  for  the  mechanism  for  the  appreciation 
of  pitch.  We  have  said  that  possibly,  in  the  cultivated 
musical  ear,  the  training  of  the  muscles  attached  •  to  the 
drum-head,  or  rather  the  recognition  of  the  muscular  sensa- 
tion caused  by  varying  degrees  of  contraction  of  these 
muscles,  may  play  a  large  part.  It  may  now  be  added 
that  this  sensation  may  be  strengthened  by  the  feeling  of 
tension  in  the  spiral  ligament  ;  but  at  present  this  is  merely 
a  conjecture. 

Auditory  Sensations 

Physiological  Characters  of  Sounds. — We  have  already 
referred  briefly  to  the  physical  causation  of  sound,  and  we 
shall  now  consider  how  the  physiological  variations  arise  in 
connection  therewith.  When  we  seek  to  analyse  the  effect 
produced  in  consciousness  by  the  stimulation  of  the  auditory 
mechanism,  we  find  that  all  sounds  may  be  roughly  divided, 
in  the  first  place,  into  such  as  we  designate  noises,  and 
those  recognised  as  musical  tones.  The  sounds  of  a  peal 
of  thunder,  of  the  rending  of  silk,  of  the  creaking  of  a  door 
on  dry  hinges — these  we  call  noises  ;  but  when  a  tuning- 
fork  vibrates,  or  a  note  on  the  piano  is  sounded,  we  call  the 
effect  produced  upon  the  ear  musical.  The  difference,  how- 
ever, between  a  noise  and  a  musical  sound  is  not  of  a  hard 
and  fast  kind.  One  may  merge  insensibly  into  the  other. 
The  tuning  of  musical  instruments  by  an  orchestra  gives  us 


Sound  and  Hearing  241 

a  noise  as  result,  but  the  noise  is  made  up  of  musical  tones, 
and  many  sounds  usually  dismissed  as  noises,  such  as 
street  calls,  the  barking  of  dogs,  or  the  blast  of  a  fog-horn, 
contain  a  distinctly  musical  element.  When  aerial  vibra- 
tions agitate  the  ear  in  regular  recurrence,  when  equal 
periods  of  time  elapse  between  each  stimulation,  the  sound 
produced  is  musical ;  but  in  the  example  mentioned  above, 
of  the  sound  produced  when  an  orchestra  tunes  its  instru- 
ments, the  musical  tones  from  the  different  players  come  at 
irregular  intervals,  and  at  rates  which  interfere  with  one 
another  in  such  a  way  as  to  produce  a  harsh  or  unmusical 
sound.  On  the  other  hand,  sounds  professedly  musical  are 
sometimes  noises  of  the  most  disagreeable  nature.  As  a 
combination  of  musical  tones  may  produce  a  noise,  we  will 
best  arrive  at  a  clear  comprehension  of  auditory  sensations 
in  general  by  the  study  in  the  main  of  musical  sounds. 

Apart  from  the  emotional  feelings  which  may  be  aroused 
by  music,  there  are  certain  sensations  produced  in  the 
mind  on  hearing  a  musical  tone.  These  sensations  may 
be  divided  under  three  heads  — first,  of  pitch  ;  second,  of 
inte?isity ;  and  third,  a  sensation  of  a  special  quality  of  the 
sound,  dependent  upon  whether  it  is  one  simple  sound,  or 
a  combination  of  simple  sounds.  In  practice,  we  seldom 
hear  simple  musical  tones,  such  as  are  produced  by  a 
tuning-fork.  The  sounds  produced  by  such  musical  instru- 
ments as  the  piano,  violin,  or  flute,  are  not  simple  tones, 
but  sounds  in  which  many  simple  tones  are  blended  into 
one  so  as  to  give  a  sound  with  a  special  quality,  timbre,  or 
klang,  by  which  we  can  recognise  the  kind  of  instrument 
that  has  given  it  forth.  But,  given  the  pitch,  intensity, 
and  quality  of  a  sound,  we  can,  with  proper  instruments, 
reproduce  any  variety  of  tone  we  please.  We  shall  con- 
sider, then,  in  the  first  place,  the  nature  of  pitch  and  of 
intensity  or  loudness,  and  then  how  tones  of  varying  pitch 

R 


242  Physiology  of  the  Senses 

and  intensity  combine  to  give  rise  to  a  sensation  of  quality 
in  a  musical  tone. 

i.  Pitch. — The  pitch  of  a  tone  depends  upon  the  fre- 
quency of  the  vibrations  in  a  given  time  ;  or,  to  put  it  in 
another  way,  since  the  wave-length  is  shorter  in  direct  pro- 
portion to  the  rapidity  of  recurrence,  the  pitch  depends 
upon  the  length  of  the  waves  which  go  to  produce  the 
sound.  If  the  vibrations  come  too  slowly  or  too  rapidly, 
no  musical  sound  is  perceived,  and  while  ears  may  hear 
musical  tones  produced  by  vibrations  at  rates  varying  from 
about  30  to  40,000  per  second,  the  range  of  the  tones 
employed  in  music  lies  between  30  and  4000  per  second. 

The  fact  that  pitch  depends  upon  frequency  of  vibration 
can  be  easily  demonstrated  by  means  of  an  instrument 
called  the  syren.  This,  in  its  simplest  form,  is  a  thin  metal 
plate  revolving  upon  an  axle  at  a  rate  which  can  be  exactly 
regulated.  The  plate  is  perforated  by  a  set  of  holes  at 
equal  distances  from  the  axle  and  from  one  another.  The 
wheel  is  first  caused  to  rotate  slowly,  and  a  current  of  air  is 
blown  against  the  plate,  so  that  it  will  pass  through  the 
holes  when  they  pass  a  certain  point.  At  first  a  series  of 
puffs  is  heard,  but,  as  the  speed  of  rotation  is  gradually 
increased,  the  puffs  begin  to  coalesce,  and  when  they  recur 
at  from  20  to  30  times  a  second,  a  low  buzzing  or  droning 
sound  is  heard.  The  faster  the  plate  revolves,  the  more 
numerous  the  puffs  become,  and  the  higher  will  be  the 
pitch,  until  at  last  the  sound  grows  faint  and  ceases  to  be 
audible.  When  the  pitch  of  a  sound  is  very  high,  the 
effect  produced  upon  the  listener  is  unpleasant.  It  is  as  if  a 
thin  metallic  blade  or  needle  were  piercing  the  ears,  or  it 
may  be  compared  to  the  shimmering  effect  of  sunlight  re- 
flected by  the  ripplets  on  the  surface  of  water  agitated  by  a 
light  breeze.  If  the  plate  be  made  to  rotate  quickly  and  at 
constant  speed,  the  pitch  of  the  note  will  remain  the  same. 


Sound  and  Hearing 


243 


Von  Helmholtz  has  devised  a  double  syren,  with  which  many 
interesting  experiments  can  be  performed  as  to  the  nature 

d 


Fig.  124. — Double  Syren  of  von  Helmholtz.  «o>  #l>  Brass  wind-chests  com- 
municating by  tubes,  g$,  g\,  with  bellows  ;  the  opposite  ends  of  the  cylinders 
are  closed  by  brass  plates  perforated  with  holes  corresponding  to  those  seen 
in  the  disk,  Cq  ;  the  disks,  cq,  c\,  rotate  on  a  common  axis,  k,  provided  with  a 
screw  for  the  counting  apparatus,  which  is  omitted  here.  The  upper  cylinder, 
a,  can  be  rotated  on  a  vertical  axis  in  either  direction  by  toothed  wheel,  e, 
with  handle,  d;  the  four  rows  of  holes  may  be  opened  or  shut  by  means  of 
studs,  i,  i;  there  are  8,  10,  12,  and  18  holes  respectively  in  the  four  rows  of 
holes  in  the  lower  disk,  and  9,  12,  15,  and  16  in  the  upper  (not  seen  in 
diagram). 

of  pitch.      It  consists  (Fig.  124)  of  two  boxes,  supplied  by 
bellows  with  air,  which,  emerging  through  the  lids  of  the 


244  Physiology  of  the  Senses 

boxes  by  holes,  the  number  of  which  can  be  varied,  causes 
a  plate  close  to,  and  in  a  parallel  plane  with,  the  lid  of  each 
box  to  rotate.  The  rotation  of  the  parallel  plates  allows 
the  air  to  escape  through  several  series  of  holes  in  them, 
just  as  in  the  simple  syren.  The  beauty  of  the  mechanism 
lies  in  the  power  it  gives  us  of  regulating  exactly  the  num- 
ber of  impulses  per  second,  of  reading  off  the  number  upon 
a  dial,  and  of  permitting  us  to  note  the  effects  produced 
when  the  two  syrens  are  emitting  tones  of  different  pitch. 
It  is  thus  most  valuable  in  studying  concords,  discords,  and 
beats,  the  nature  of  which  will  be  described  shortly.  One 
point  which  invariably  arrests  the  attention  when  the  syren  is 
heard  for  the  first  time  is  the  peculiar  effect  of  the  gradual 
rise  in  pitch  as  the  velocity  of  rotation  is  accelerated.  We 
may  say  that  at  one  moment  it  is  giving  forth  many  im- 
pulses, say,  200  per  second ;  at  another  a  different  num- 
ber, say,  201  ;  but  the  change  from  200  to  201  is  through 
an  infinite  fractional  series  ;  and  so  with  regard  to  the  sound  ; 
it  does  not  rise  by  leaps  and  bounds,  but  glides  up  in  con- 
tinuous transition.  Just  as  the  colours  of  the  spectrum  vary 
through  an  infinite  series,  in  passing  from  one  colour  to 
another,  so  do  the  sounds  in  changing  from  one  pitch  to 
another.  The  same  effect  can  be  produced  on  the  violin 
by  sliding  the  finger  up  the  string  while  it  is  being  bowed. 
And,  further,  as  has  been  mentioned  with  regard  to  per- 
ception of  colour,  as  some  eyes  are  insensible  to  the  red, 
and  others  to  the  violet  end  of  the  spectrum,  so  some  ears 
are  insensitive  to  sounds  of  low  pitch,  others  to  those  of 
high  pitch.  As  might  naturally  be  expected,  the  sensibility 
to  pitch  varies  more  in  the  higher  than  in  the  lower  parts 
of  the  scale,  and  we  find  people  who  suppose  their  powers 
of  hearing  to  be  perfectly  normal,  who  yet  fail  to  hear 
sounds  due  to  more  than  6000  vibrations.  Test  of  power 
in  this  respect  may  be  made  by  means  of  a  set  of  short- 


Sound  and  Hearing  245 

steel  cylinders,  made  by  Konig,  which,  when  suspended 
by  threads  to  a  wooden  frame,  and  struck  with  a  metallic 
instrument,  emit  tones  to  upwards  of  40,000  vibrations  per 
second.  The  same  result  may  be  attained  by  using  short- 
limbed  or  heavy  tuning-forks. 

Within  the  range  of  musical  pitch,  too,  we  find  that 
people  vary  much  in  their  capability  of  distinguishing  a  tone 
of  one  pitch  from  another  nearly  the  same.  This  likewise 
holds  good  in  respect  of  colour.  Orientals  distinguish 
many  shades  of  colours  which  seem  the  same  to  us. 
While  most  people  can  detect  a  difference  of  a  semitone  in 
two  notes  sounding  together  when  of  medium  pitch,  some 
acute  ears  can  detect  as  small  a  difference  as  -jA-th  of  a 
semitone.  It  becomes  more  and  more  difficult  to  detect 
the  difference  as  we  pass  to  the  upper  or  lower  limits  of 
hearing — a  fact  one  may  readily  prove  for  oneself  by  striking 
adjoining  keys,  now  in  the  centre,  now  at  either  end  of  the 
key-board  of  a  piano.  We  have  already  indicated  that  the 
power  of  detecting  variations  in  pitch  can  be  increased  by 
exercise  and  training,  and  have  suggested  a  possible 
explanation  as  to  how  this  is  so.  On  the  other  hand,  there 
are  some  people  who  are  unable  to  discriminate  more  than 
a  very  few  tones,  and  who  find  it  utterly  impossible  to  sing 
any  complicated  tune.  The  pitch  of  the  ordinary  human 
voice  in  singing,  it  may  be  mentioned  in  passing,  may  be 
as  low  as  ia.Y  (S7  vibrations  per  second),  or  as  high  in  a 
good  soprano  as  sol4  (768  vibrations  per  second) ;  or,  in 
other  words,  it  is  comprised  within  a  range  of  a  little  more 
than  three  octaves.  There  have  been  a  few  exceptional 
singers  who  have  been  able  to  sing  pure  musical  notes  be- 
yond these  limits.  Thus  Gaspard  Forster,  a  basso,  passed 
from  f a  - 1  (42  vibrations)  to  la3  (435  vibrations) ;  it  is  said 
that  Nilsson,  in  77  Flanto  Alagico,  can  take  fa5  (1365  vibra- 
tions) ;  and  Mozart  states  that  in  Parma,  in  1770,  a  soprano, 


246  Physiology  of  the  Senses 

Lucrezia  Ajugari,  ranged  from  sol2  (192  vibrations)  to  do(, 
(2048  vibrations).  The  latter  is  the  most  highly  pitched 
voice  in  musical  literature,  an  octave  and  a  half  above  the 
highest  ordinary  soprano.  The  extreme  range  of  the 
human  voice,  then,  taking  into  account  the  extraordinary 
voices  above  alluded  to,  is  from  fa  —  x  (42  vibrations)  to 
do6  (2048  vibrations),  or  about  six  octaves,  while  the  range 
of  the  human  ear  for  musical  tones  is  from  do  —  1  (32  vibra- 
tions) to  do10  (nearly  40,000  vibrations),  or  about  eleven 
octaves. 

2.  Intensity  or  Loudness. — The  second  character  of 
a  musical  tone  which  we  notice  is  its  intensity  or  loud- 
ness. This  varies  with  the  amplitude  of  vibration  of  the 
sounding  body.  Thus  a  tuning-fork  bowed  gently  will  give 
out  a  faint  sound,  while  the  same  fork  bowed  strongly  will 
give  a  note  of  the  same  pitch  as  the  former,  but  sotinding 
much  louder. 

In  the  case  where  the  particles  of  the  wave  move  at 
right  angles  to  the  direction  in  which  the  wave  is  advancing, 
as,  for  instance,  a  wave  on  the  surface  of  water,  one  can 
readily  understand  what  is  meant  by  the  height  or  amplitude 
of  the  wave.  But  this  is  not  so  easy  in  connection  with  a 
wave  of  sound  where  the  particles  are  moving  in  the  same 
direction  as  the  wave,  and  we  are  apt  to  confuse  the  ampli- 
tude with  the  length  of  the  wave,  which,  as  we  have  seen, 
is  invariable  in  any  given  medium  for  any  given  note,  and 
determines  pitch,  not  intensity.  We  can  probably  realise 
the  meaning  of  amplitude  best  in  connection  with  sound 
waves  by  thinking  of  what  happens  when  a  large  tuning- 
fork  is  vibrating  feebly  or  strongly.  In  the  one  case,  the 
excursion  of  the  limbs  is  so  small  that,  to  the  unaided  eye, 
the  fork  seems  to  be  motionless  ;  in  the  other,  there  is  a 
perceptible  movement  through  space,  and  though  the  pitch 
of  the  note  remains    the   same,   it   has    a   louder,  stronger 


Sound  and  Hearing  247 

effect  upon  the  ear.  The  fork  makes  exactly  the  same 
number  of  vibrations  in  each  case,  but  in  the  latter  its 
limbs  move  through  a  greater  distance.  Hence  more 
molecules  of  air  must  at  one  moment  be  crowded  into  a 
given  space,  at  another  there  must  be  a  more  complete 
rarefaction  of  the  air.  There  must  then  be  a  greater 
difference  in  the  degree  of  pressure  upon  the  drum-head  of 
the  ear ;  at  one  time  a  greater  increase,  at  the  next  a 
greater  diminution.  Corresponding  to  this,  there  will  be 
greater  movement  of  the  tympanic  ossicles,  and  more 
variation  in  the  pressure  on  the  internal  ear,  and  disturb- 
ance of  the  nervous  arrangements.  The  contrast  of  loud 
and  faint  sounds  can  be  readily  made  by  holding  to  the 
ear  a  vibrating  tuning-fork,  and  turning  it  round  between 
finger  and  thumb,  now  this  way,  now  that.  It  will  be 
found — and  this  bears  out  the  statement  just  made  as  to 
amplitude — that  the  sound  is  loudest  when  the  plane  in 
which  the  limbs  are  vibrating  is  at  right  angles  to  the  side 
of  the  head,  for  here  the  air  is  disturbed  with  the  greatest 
energy.  The  same  experiment  also  shows  the  gradual 
transition  in  intensities  just  as  in  the  case  of  pitch.  The 
more  the  energy  of  vibration,  or,  in  other  words,  the 
greater  the  number  of  molecules  packed  into  a  given  space 
in  a  given  time,  the  greater  will  be  the  loudness — a  pheno- 
menon comparable  to  the  sensation  of  varying  brightness 
of  light. 

3.  Quality,  Timbre,  Klang. — The  quality  of  a  musical 
sound  enables  us,  after  a  due  amount  of  training,  to  know, 
from  the  effect  produced  upon  the  ear,  what  is  the  instru- 
ment by  which  the  sound  has  been  produced.  We  readily 
distinguish,  for  example,  a  musical  note  produced  upon  the 
piano  from  that  of  the  violin,  or  either  of  these  from  the 
tones  of  the  human  voice,  or  of  a  wind  instrument  such  as 
the  flute.       Each  kind  of  instrument  produces    a    set    of 


248  Physiology  of  the  Senses 

characteristic  wave-forms,  and  the  musician  can  tell  by  the 
effect  produced  what  kind  of  instrument  is  sounding. 

The  simplest  form  of  vibration  which  gives  rise  to  the 
sensation  of  a  musical  tone  is  that  of  a  body  vibrating  in 
simple  harmonic  motion.  Suppose  a  disturbance  to  be 
made  in  the  perfectly  smooth  and  level  surface  of  a  sheet 
of  water.  A  concentric  series  of  waves  will  spread  out- 
wards from  the  point  of  disturbance  in  ever-widening 
circles.  But  while  the  wave -forms  move  outwards,  the 
particles  which  go  to  form  the  waves  have  only  a  vertical 
motion,  up  to  the  crest  of  the  wave  above,  or  down  into 
the  trough  below,  the  ordinary  water-level ;  and  after  a 
series  of  gradually  diminishing  oscillations,  they  come  to 
rest  exactly  in  the  position  from  which  they  started.  If 
the  waves  were  all  of  equal  size  the  particles  would  move 
up  and  down  in  simple  harmonic  motion.  Similarly,  when 
a  tuning-fork  is  vibrating  so  as  to  give  forth  a  pure  tone,  its 
various  parts  move  in  approximately  simple  harmonic 
motion.1 

If  we  attach  a  stylet  to  the  limb  of  a  tuning-fork,  set  the 
fork  vibrating,  and  allow  the  stylet  to  write  upon  a  sheet  of 
paper  drawn  in  the  direction  of  the  length  of  the  fork,  a 
curved  line  will  be  traced  upon  the  paper  similar  to  the 
curve  from  dQ  to  8  in  Fig.  125.  The  shape  of  the  tracing 
will  depend  upon  the  rate  at  which  the  paper  moves.  If 
the  paper  moves  slowly  the  waves  will  be  short  and  steep  ; 
if  quickly,  they  will  be  elongated.  Such  a  series  of  vibra- 
tions reaching  the  ear  gives  rise  to  a  sensation  which,  lacking 

1  A  simple  harmonic  motion  is  thus  mathematically  defined  by 
Thomson  and  Tait,  Elements  of  Nat.  Phil.  Part  I.  p.  19  :  "  When  a 
point  Q  moves  uniformly  in  a  circle,  the  perpendicular  QP  drawn  from 
its  position  at  any  instant  to  a  fixed  diameter  AAofthe  circle,  intersects 
the  diameter  in  a  point  P,  whose  position  changes  by  a  simple  harmonic 
motion." 


Sound  and  Hearing 


249 


brilliancy  and  variety,  soon  palls  on  the  ear.  The  one 
continuous  tone  has  a  dull  uniformity  ;  it  is  monotonous  in 
every  sense  of  the  word. 

In  the  next  place,  suppose  we  have  two  tuning-forks 
vibrating  at  the  same  time  but  at  different  rates,  and  for 
the  sake  of  simplicity  let  one  of  them  vibrate  twice  as 
quickly  as  the  other.  We  can  now  attend  at  will  to  the 
tone  given  forth  by  either  fork,  or  to  a  new  third  sensation 


Fig.  125. — Pendular  vibrational  curves  A  and  B.  C,  Vibrational  curve  obtained 
by  superimposing  B  on  A,  so  that  the  point  e  is  on  do  ;  D,  vibrational  curve 
obtained  by  superimposing  B  on  A,  with  the  point  e  on  d\  of  A.  (Von 
Helmholtz.) 


produced  by  the  combination  of  the  two  tones.  If  the 
waves  of  condensation  begin  at  exactly  the  same  instant,  the 
combined  effect  may  be  graphically  represented  by  the 
continuous  line  in  C,  Fig.  125.  When  both  forks  produce 
condensation  or  rarefaction  of  the  air  at  the  drum-head  at 
the  same  time,  the  effect  will  be  that  of  the  sum  of  the  two. 
If  one  tend  to  produce  condensation,  while  the  other 
causes  rarefaction,  the  combined  effect  will  be  equal  to  the 


250  Physiology  of  the  Senses 

difference  of  the  two.  Thus  the  height  of  the  continuous 
curve  C  (Fig.  1 25)  at  the  perpendicular  cY  is  equal  to  the  sum 
of  the  height  aY  dx  of- wave  A,  and  of  the  height  of  the  crest 
at  bx  in  curve  B.  At  d2  no  effect  is  produced  by  B  as  the 
crest  is  changing  to  the  trough.  At  a2  d3,  A  is  still  pro- 
ducing condensation,  while  B  is  producing  rarefaction,  the 
resultant  effect  being  that  at  this  phase  the  continuous 
line  c  falls  below  the  dotted  line  between  c^  c2,  and  so  on. 
If  the  crests  do  not  occur  at  the  same  moment,  but  at 
different  times,  as  in  D,  the  resultant  form  of  wave 
will  be  different  from  that  of  C.  Similarly  in  the  case 
of  the  smooth  sheet  of  water,  if  the  surface  be  disturbed 
at  two  points  the  waves  meeting  and  intersecting  will 
have  increased  height  or  depth  when  crest  meets  crest 
or  when  trough  meets  trough,  but  if  the  crest  of  the  one 
coincide  with  the  trough  of  the  other,  the  measure  of 
the  amplitude  of  the  resultant  wave  will  be  the  difference 
between  the  two.  If  the  waves  be  of  the  same  size  and 
meet  so  that  the  crest  of  one  exactly  coincides  with  the 
trough  of  the  other,  they  will  counterbalance  or  neutralise 
each  other,  and  the  result  will  be  a  level  surface  for  the 
water,  or  in  the  case  of  sonorous  vibrations  rest  of  the 
molecules  and  silence.  And  now  let  us  suppose  that  we 
have  an  indefinite  number  of  sets  of  vibrations,  whose 
period  or  time  of  vibration  is  such  that  the  primary  or 
fundamental  series  is  always  a  multiple  of  the  smaller  or 
more  rapid  sets,  then  the  resultant  curves,  as  graphically 
represented,  may  assume  an  infinite  variety  of  forms,  but 
these  being  repeated  at  regular  intervals,  the  effect  upon 
the  ear  will  be  that  of  a  musical  note.  What  complicated 
forms  the  wave  may  take  can  be  readily  imagined  if  we 
think  of  the  effect  produced  on  the  surface  of  the  sea  by  a 
gale  of  wind.  The  great  rollers  have  their  crests  buffeted 
and  broken  by  conflicting  gusts,  their  surfaces  roughened 


Sound  and  Hearing 


251 


by  a  thousand  waves  and  ripplets.  No  two  great  waves 
seem  exactly  alike.  Such  a  disturbance  of  the  atmosphere 
affecting  the  ear  would  give  rise  simply  to  a  noise,  but  let 
the  great  waves,  irregular  as  they  may  be,  succeed  each 
other  as  exact  copies  one  of  the  other,  then  we  will  have  the 
musical  tone,  whose  pitch  or  fundamental  tone  is  that  of 
the  largest  waves,  but  whose  quality  is  determined  by  the 
combination  of  waves  and  wavelets  into  one. 

Resonators. — We  can  easily  prove  that  the  musical 
notes  of  most  instruments  are  compounded  of  a  fundamental 
and  upper  partial  or  overtones  by  using  the  resonators 
of  von  Helmholtz.  These  are  hollow  spheres  of  brass  or 
glass  with  apertures  to  either  side,  as  seen  in  Fig.  126,  or 
tubes  shaped  somewhat  like  a  bottle  with- 
out a  bottom.  The  air  in  these  instru- 
ments vibrates  at  a  given  rate,  or  in 
other  words,  with  a  certain  pitch  deter- 
mined by  the  size  of  the  resonator  (the 
larger  the  resonator  the  lower  the  pitch), 
and  most  loudly  when  a  note  of  the 
same  pitch  is  sounded  in  the  vicinity 
of  the  resonator.  When  the  smaller  aperture  is  inserted 
into  the  external  ear  the  special  tone  is  heard  to  the  exclu- 
sion of  all  others,  the  amplitude  of  the  vibration  being 
largely  increased  in  the  resonator.  The  principle  by  which 
this  is  brought  about  is  the  same  as  that  which  comes 
into  play  when  any  periodic  motion  is  increased  in  amplitude 
by  slight  successive  increments.  For  instance,  suppose  we 
wish  to  cause  a  person  sitting  on  a  swing  to  rise  to  a  con- 
siderable height,  or,  in  scientific  terms,  to  cause  the  swing 
to  move  in  vibrations  of  large  amplitude.  We  first  push 
the  swing  from  the  vertical,  and  thereby  cause  it  to  rise 
a  slight  distance  above  its  lowest  position.  Under  the 
influence  of  gravity  the  swing  falls  back  to  its  position  of 


Fig.  126.— Resonator  of 
von  Helmholtz. 


252  Physiology  of  the  Senses 

rest,  but  acquiring  momentum  as  it  falls  it  passes  the 
vertical  line  and  rises  on  the  other  side  until  stopped  by 
gravity,  the  friction  of  the  rope,  and  the  resistance  offered 
by  the  air  to  the  movement  of  the  body  through  it.  If, 
further,  we  ourselves  interpose,  we  can  readily  prevent  the 
rise  and  bring  the  swing  to  rest.  But  suppose  we  wait 
till  the  swing,  having  risen  as  high  as  possible,  stops  and 
begins  to  fall  again  and  now  give  another  slight  push  in 
the  same  direction  as  formerly.  The  new  force  added  to 
the  old,  which  has  not  yet  entirely  died  away,  causes  the 
swing  to  rise  a  little  higher  than  at  first,  and  the  return  rise 
is  also  higher.  Again,  when  it  begins  to  fall  we  give  a  slight 
push,  and  so  on,  till  at  last  the  swing  sweeps  to  and  fro  in 
wide  oscillations  and  with  great  momentum.  The  periodic 
application  of  a  slight  force  has  given  rise  by  summation  of 
effect  to  a  great  force  and  extensive  movement.  So  is  it 
with  the  resonator.  Vibrations  of  small  amplitude  in  the 
external  air  set  the  molecules  of  air  in  the  resonator  into 
oscillation,  and  the  successive  impulses  are  given  just  at  the 
moment  when  they  will  increase  the  amplitude  of  vibration. 
Thus  atmospheric  vibrations  which,  when  diffused  freely 
through  the  air,  have  insufficient  energy  to  give  rise  to  a 
sensation,  will,  acting  upon  the  air  in  the  resonator,  set  up  a 
sympathetic  resonance,  which  enables  the  ear  to  detect 
their  presence  even  amid  a  multitude  of  louder  sounds.  But 
if  the  pitch  of  the  external  note  is  sharpened  or  flattened, 
the  vibrations  clash,  and  the  resonator  is  silent. 

Analysis  of  Compound  Tones  by  Resonators.  —  To 
satisfy  ourselves  that  the  sound  produced  by  most  musical 
instruments  is  compounded  of  many  simple  tones,  we  have 
simply  to  sound  a  note  upon  the  instrument  in  question,  and 
listen  with  a  series  of  resonators.  We  will  have,  firstly, 
resonance  for  the  fundamental  tone,  and  then  for  a  set  of 
tones    of    higher    pitch    whose    vibrational    numbers    are 


Sound  and  Hearing  253 

multiples  of  that  of  the  fundamental  tone.  We  might  have, 

for  example,  a  set  of  overtones  or  partials  or  harmonics  of 
the  following  relationship  : — 


Fundamen 

.tal 

Upper  Part: 

ials  or  ] 

Harmonics. 

Note       .     .    do1 

do2 

sol2 

do3     mi3 

sol3 

si|?3     do4     re4 

mi4 

Partial  tones      i 

2 

3 

4        5 

6 

7        8       9 

10 

Number    of )    „ 

r  33 
vibrations  J 

66 

99 

132     165 

198 

231    264   297 

330 

Instead  of  applying  a  series  of  resonators  to  the  ear, 
and  so  detecting  the  presence  of  various  simple  tones  by 
hearing,  we  may  analyse  the  compound  note,  and  demon- 
strate optically  the  presence  of  the  partial  tones  by  means 
of  an  apparatus  devised  by  Konig.  This  consists  of  a 
series  of  resonators  mounted  on  a  frame.  The  apertures  of 
the  resonators,  which  are  usually  inserted  into  the  ear,  are 
connected  by  elastic  tubing  with  a  set  of  small  boxes. 
Coal-gas  is  led  into  the  boxes,  but  prevented  from  passing 
to  the  resonators  by  closure  of  the  entrance  to  the  tubes 
with  a  thin  india-rubber  membrane.  The  gas  passes  from 
the  boxes  to  a  corresponding  set  of  small  burners,  which 
give  long  pointed  flames.  When  the  air  in  one  of  the 
resonators  is  set  in  vibration,  the  membrane  shutting  off  the 
resonator  from  the  gas-box  vibrates  in  sympathy,  causing 
a  variation  in  the  pressure  of  the  gas,  and  of  the  size  of  the 
flame.  With  all  musical  tones,  however,  the  number  of 
vibrations  per  second  is  so  great  that,  from  persistence  of 
the  retinal  impression,  we  are  unable  with  the  naked  eye  to 
see  the  change  in  size  of  the  flame.  To  obviate  this  diffi- 
culty, the  rays  of  light  from  the  flame  are  reflected  to  the 
eye  from  the  surface  of  a  cubical  mirror  rotating  upon  an 
upright  axis.  If  the  flame  is  burning  steadily,  the  series  of 
reflections  of  the  light  sent  from  the  rotating  mirror  are 
blended  into  one  smooth  edged  band  of  light ;  but  if  the 


254 


Physiology  of  the  Senses 


resonator  is  in  action,  the  smooth  band  gives  place  to  one 
with  teeth  on  its  upper  border.  Each  tooth  represents  an 
increase  of  pressure  from  the  resonator,  each  notch  a  diminu- 
tion. When  a  note  containing  the  overtones  to  which  these 
resonators  respond  is  sounded,  the  flame  picture  in  the 
mirror  will  declare  their  presence.     The  adaptation  to  organ 


Fig.  127. — Konig's  apparatus  for  studying  optically  the  vibration  of  air  in 
organ-pipes. 

pipes  of  the  same  kind  of  apparatus,  viz.  the  gas-box,  and 
the  light  of  the  flame  reflected  from  a  rotating  mirror,  is 
shown  in  Fig.  127,  where  we  have  the  means  of  studying 
the  vibration  of  air  in  organ  pipes.  By  such  an  arrange- 
ment, for  example,  we  may  see  that  with  two  organ  pipes 
sounding,   the  one  an   octave  higher  than   the  other,   the 


Sound  and  Hearing  255 

flame  picture  on  the  mirror  for  the  upper  note  will  have 
twice  as  many  elevations  as  that  of  the  lower. 

In  the  absence  of  von  Helmholtz's  resonators,  a  simple 
means  of  analysing  a  compound  note,  or  at  least  of  detect- 
ing its  most  important  partial  tones,  is  to  cause  the  note  to 
sound  beside  a  piano.  If  we  gently  depress  the  key  corre- 
sponding in  pitch  to  that  of  the  note  sounded,  so  as  to 
remove  the  damper,  we  will  hear  quite  distinctly  the  sound 
of  the  piano-string  vibrating  in  sympathetic  resonance.  Next 
depress  the  key  of  the  octave  above,  and  we  will  hear  it 
sounding,  but  more  faintly  than  the  fundamental  note. 
Again,  if  we  press  down  the  key  of  the  fifth  (sol)  in  the 
second  octave,  and  so  on  with  the  various  harmonic  over- 
tones, we  will  hear  the  resonance,  but  always  becoming 
weaker.  It  will,  as  a  rule,  be  found  that  the  sound  obtained 
from  any  note  other  than  those  in  the  harmonic  series  is 
by  no  means  so  distinctly  heard,  although  we  may  have  in- 
harmonic upper  tones  due  to  a  note  being  not  purely  musical 
in  character,  but  accompanied  in  its  production  or  propaga- 
tion by  noises. 

For  the  notes  sounded  by  almost  all  musical  instruments, 
then,  we  may  conclude  that  each  note  is  compounded  of  a 
series  of  simple  tones,  each  of  which  may  be  made  to  pro- 
duce its  effect  upon  the  ear  as  if  the  others  were  absent, 
and  the  total  effect  is  due  to  a  summation  of  the  effects  and 
a  combination  thereof  to  give  a  new  sensation. 

We  can  imitate  the  notes  of  instruments  having  special 
overtones  by  combining  pure  partial  tones,  and  in  the  organ 
some  of  the  stops  are  so  designed  as  to  make  sets  of  pipes 
sound  together  whose  pitch  is  such  as  to  give  the  effect 
of  some  other  instrument,  such  as  the  flute,  the  clarinet,  or 
even  the  human  voice  (the  vox  humand). 

As  a  result  of  a  careful  series  of  observations  on  the 
quality  of  different  musical  tones,  the  particulars  of  which 


256  Physiology  of  the  Senses 

are  detailed  in  his  book   On  the  Sensations  of  Tone,1  von 
Helmholtz  arrives  at  the  following  conclusions  : — 

"  1.  Simple  tones,  like  those  of  tuning-forks  applied  to 
resonance  chambers,  and  wide  stopped  organ  pipes,  have  a 
very  soft  pleasant  sound,  free  from  roughness,  but  wanting 
in  power,  and  dull  at  low  pitches. 

"2.  Musical  tones,  which  are  accompanied  by  a  moder- 
ately loud  series  of  the  lower  upper  partial  tones  up  to 
about  the  sixth  partial,  are  more  harmonious  and  musical. 
Compared  with  simple  tones  they  are  rich  and  splendid, 
while  they  are  at  the  same  time  perfectly  sweet  and  soft 
if  the  higher  upper  partials  are  absent.  To  these  belong 
the  musical  tones  produced  by  the  pianoforte,  open  organ 
pipes,  the  softer  piano  tones  of  the  human  voice,  and  of  the 
French  horn.  The  last-named  tones  form  the  transition  to 
musical  tones  with  high  upper  partials  ;  while  the  tones  of 
flutes,  and  of  pipes  on  the  flute  stops  of  organs,  with  a  low 
pressure  of  wind,  approach  to  simple  tones. 

"3.  If  only  the  uneven  partials  are  present  (as  in  narrow 
stopped  organ  pipes,  pianoforte  strings  struck  in  their  middle 
points,  and  clarinets)  the  quality  of  tone  is  hollow,  and, 
when  a  large  number  of  such  upper  partials  is  present,  nasal. 
When  the  prime  tone  predominates,  the  quality  of  tone  is 
rich  and  full ;  but  when  the  prime  tone  is  not  sufficiently 
superior  in  strength  to  the  upper  partials,  the  quality  of 
tone  is  poor  or  empty.  Thus  the  quality  of  tone  in  the 
wider  open  organ  pipes  is  fuller  than  that  in  the  narrower  ; 
strings  struck  with  pianoforte  hammers  give  tones  of  a 
fuller  quality  than  when  struck  by  a  stick,  or  pulled  by  the 
finger ;  the  tones  of  reed  pipes,  with  suitable  resonance 
chambers,  have  a  fuller  quality  than  those  without  resonance 
chambers. 

"4.   When  partial  tones  higher  than  the  sixth  or  seventh 
1  Von  Helmholtz,  Sensations  of  Tone,  pp.  172,  173. 


Sound  and  Hearing  257 

are  very  distinct,  the  quality  of  tone  is  cutting  and  rough. 
The  reason  for  this  lies  in  the  dissonances  which  they  form 
with  one  another.  The  degree  of  harshness  may  be  very 
different.  When  their  force  is  inconsiderable,  the  higher 
upper  partials  do  not  essentially  detract  from  the  musical 
applicability  of  the  compound  tones  ;  on  the  contrary,  they 
are  useful  in  giving  character  and  expression  to  the  music. 
The  most  important  musical  tones  of  this  description  are 
those  of  bowed  instruments,  and  of  most  reed  pipes,  oboe 
(hautbois),  bassoon  (fagot),  physharmonica  (harmonium, 
concertina,  accordion),  and  the  human  voice.  The  rough 
braying  tones  of  brass  instruments  are  extremely  penetrat- 
ing, and  hence  are  better  adapted  to  give  the  impression  of 
great  power  than  similar  tones  of  a  softer  quality.  They 
are  consequently  little  suitable  for  artistic  music  when  used 
alone,  but  produce  great  effect  in  an  orchestra." 

It  has  been  stated  that  the  quality  of  a  tone  is  dependent 
upon  the  form  of  the  wave  which  produces  it.  We  have 
seen  that  the  graphic  representation  of  a  complex  tone 
reveals  a  series  of  very  different  forms  of  waves,  according 
to  the  phase  or  period  of  combination  of  the  partial  tones. 
The  question  then  arises  :  Does  the  ear  appreciate  these 
differences  of  phase  in  the  combinations  of  partial  tones  ? 
For  a  given  set  of  combined  partial  tones,  do  the  different 
resultant  wave-forms  give  rise  to  sensations  of  different 
quality  ?  To  this  question  conflicting  answers  have  been 
given.  On  the  one  hand,  it  is  maintained  by  von  Helm- 
holtz  that  "  the  quality  of  the  musical  portion  of  a  compound 
tone  depends  solely  on  the  number  and  relative  strength 
of  its  partial  simple  tones,  and  in  no  respect  on  their  differ- 
ences of  phase,"  The  difference  of  wave-forms  C  and  D  in 
Fig.  125,  according  to  von  Helmholtz,  makes  no  difference  in 
the  sensation  of  the  quality  of  the  resultant  complex  tone. 
The  ear  has  the  power  of  resolving  the  complex  vibrations 

S 


258  Physiology  of  the  Senses 

into  series  of  simple  vibrations,  and  of  hearing  the  pure 
tones  corresponding  to  these  sets  of  vibrations.  As  accord- 
ing to  mathematical  demonstration,  however  different  the 
wave -forms  for  any  given  combinational  tone  may  be, 
varying  with  phase  of  combination,  these  forms  can  only 
be  resolved  into  one  definite  set  of  partial  tones,  the  ear 
must  always  recognise  the  same  set  of  partials,  and  we  com- 
bine them  again  to  give  rise  to  a  tone  of  the  same  quality. 
On  the  other  hand,  it  is  asserted  that  the  different  forms, 
representing  as  they  do  real  differences  in  pressure  on  the 
drum-head  of  the  ear,  give  rise  to  sensations  of  different 
quality.  The  curve  D,  for  example,  in  Fig.  125,  may  be 
taken  as  representing  short  periods  of  increased  pressure 
and  long  periods  of  diminished  pressure  upon  the  tym- 
panic membrane,  while,  by  slightly  altering  the  phase  ot 
the  component  parts,  we  could  give  rise  to  alternate  long 
periods  of  increased  pressure  and  short  periods  of  dimin- 
ished pressure.  In  the  one  case,  the  general  condition 
is  one  of  diminished  pressure  on  the  sensory  apparatus 
with  brief  change  to  high  pressure  ;  in  the  other,  the  sen- 
sory apparatus  is  subject  in  the  main  to  higher  pressure 
than  usual,  but  with  short  periods  of  low  pressure  interven- 
ing. The  pitch  and  intensity  are,  of  course,  unaffected, 
because  the  rate  of  vibration  and  amplitude  of  the  waves 
are  the  same.  The  decision  between  the  opposing  opinions 
can  be  made  only  by  personal  trial,  for,  theoretically,  we 
have  no  knowledge  as  to  the  way  in  which  variations  of 
pressure  in  the  internal  ear  affect  the  sensory  apparatus, 
nor,  again,  how  changes  in  the  end  organ  are  transmuted 
into  conscious  sensation.  As  a  matter  of  fact,  the  differ- 
ences of  quality,  if  any  do  arise,  are  very  slight,  and  only 
to  be  appreciated  by  a  highly-trained  ear,  and  with  simple 
binary  compounds.  For  the  notes  of  ordinary  musical 
instruments,  or  for  combinations  of  numerous  partials  into 


Sound  and  Hearing  259 

complex  tones,  it  is  practically  impossible  to  detect  differ- 
ences of  phase,  so  that  the  statement  holds  good  in  the 
main  that  the  quality  depends,  as  von  Helmholtz  asserts, 
upon  the  number  and  relative  strength  of  the  partial  tones. 
This  holds  for  all  perfect  harmonies,  at  least  those  in  which 
the  vibrations  are  strictly  periodic  and  resolvable  into  series 
of  partial  tones — the  period  oi  the  fundamental  tone  being  a 
multiple  of  those  of  the  partial  tones. 

Beats. — When  two  simple  tones  of  exactly  the  same 
pitch  are  sounded  together,  if  some  arrangement  be  made 
by  which  the  phase  of  vibration  of  each  coincides,  the 
result  of  their  combination  will  be  increased  amplitude 
of  vibration  of  the  drum-head,  and  increased  intensity  of 
sound,  but  if  the  phase  of  one  series  of  vibrations  differ  by 
a  half  wave-length  from  the  other,  the  one  will  neutralise 
or  interfere  with  the  other,  and  there  will  be  silence.  Sup- 
pose, now,  that  we  have  two  simple  tones  sounding  together 
of  the  same  intensity,  and  of  nearly  the  same  pitch — say, 
for  example,  that  one  is  due  to  200  the  other  to  201  vibra- 
tions per  second — and  suppose  that  the  vibrations  are  in 
the  same  phase  to  begin  with,  it  is  evident  that,  since  one 
falls  behind  the  other  to  the  extent  of  one  wave-length  in  a 
second,  it  must  fall  one-half  of  a  wave-length  behind  in  half 
a  second  ;  near  the  beginning  and  near  the  end  of  the 
second  the  vibrations  are  nearly  in  the  same  phase,  and 
combine  to  intensify  the  effect ;  but  in  the  middle  of  the 
second,  being  in  opposite  phases,  they  tend  to  counteract 
each  other,  and  there  will  be  a  diminution  of  intensity  even 
to  momentary  silence.  There  will  thus  be  an  increase  of 
volume  followed  by  a  diminution  of  volume  of  the  sound 
every  second,  and  we  have  an  unevenness  in  the  sound,  or 
a  succession  of  what  have  been  called  beats.  The  number 
of  beats  per  second  will  depend  upon,  and  be  equal  to,  the 
difference  of  rate  of  vibration  of  the  two  partial  tones.     We 


260  Physiology  of  the  Senses 

have  seen  that  a  difference  of  one  vibration  per  second  gives 
one  beat  per  second.  If  the  simple  tones  differ  by  two 
vibrations  per  second,  there  must  be  two  beats  per  second  ; 
for,  since  the  one  set  falls  two  wave-lengths  behind  the 
other  in  a  second,  they  must  be  one  wave-length  behind  in 
half  a  second,  and  a  half  wave-length  behind  in  a  quarter 
of  a  second.  There  is  increase  of  sound  about  the  begin- 
ning of  the  first  and  third  quarters,  and  diminution  about 
the  beginning  of  the  second  and  fourth  quarters,  or,  as  we 
have  said,  two  beats  per  second.  Beats,  then,  can  arise  only 
when  the  vibrational  number  of  one  set  is  not  a  multiple  of 
the  other ;  if  the  period  of  one  is  a  multiple  of  the  period 
of  the  other,  there  can  be  no  beat.  When  there  are  not 
more  than  five  or  six  beats  per  second,  the  ear  can  easily 
note  the  gradual  rise  and  fall  in  intensity,  and  the  effect  is 
not  unpleasant.  When  the  beats  come  more  quickly  we 
lose  the  power  of  paying  attention  to  the  rise  and  fall  of 
each  beat,  although  we  can  still  for  a  time  recognise  the 
beats  as  arising  and  differing  from  the  continuous  tones. 
The  effect  is  that  of  a  whirring  harsh  sound  ;  it  is  called  dis- 
sonance. According  to  von  Helmholtz,  by  gradually  increas- 
ing the  frequency  of  the  beats,  we  may  have  as  many  as  132 
per  minute,  and  yet  recognise  the  dissonant  character  of  the 
sound  and  the  presence  of  beats.  Beyond  this  number  the 
regular  recurrence  of  the  beats  leads  to  a  secondary  fusion, 
and  the  starting  of  a  new  tone  arising  from  the  beats — a  beat- 
tone.  The  ear  fails  to  recognise  a  strictly  musical  character 
in  beat-tones  even  when  the  beats  are  much  more  numerous 
than  the  vibrations  required  for  an  ordinary  musical  tone. 
This  we  may  possibly  explain  by  the  fact  that  the  develop- 
ment of  beats  is  due  not  so  much  to  a  variation  of  pitch  as 
of  intensity.  The  higher  tone  continues  to  sound  at  exactly 
the  same  pitch  as  before,  and  there  is  merely  a  periodical 
variation  in  the  amplitude  of  the  vibrations  which  give  rise 


Sound  and  Hearing  261 

to  it.     We  have,  then,  in  the  production  of  beats,  a  condi- 
tion analogous  to  the  variations  of  pressure  experienced  in 
the  sense  of  touch,  in  which,  as  stated  (p.  58),  we  are  able 
to  discriminate  the  individual  stimuli  much  longer  than  we 
can  either  with  visual  or  ordinary  auditory  stimuli.     There 
may  be  no  fusion  by  the  sense  of  touch  of  as  many  as  500 
stimuli  per  second  ;  whereas,  if  the  stimuli  to  the  eye  come 
faster  than    10  per  second,  or  by  the  ear  30  per  second, 
there  is  a  fusion  in  sensation.      In  the  phenomena  of  beats, 
then,  we  seem  to  find  a  link  between  the  sensation  of  touch 
and  that  of  hearing,  the  tactile  element  (variation  of  ampli- 
tude)  being   superposed  upon  the  auditory  element  (con- 
stancy  of  pitch).      The    unpleasantness   of  the    sensation 
excited  when  the  beats  come  at  about  35  per  second,  when 
carefully  investigated,  is  found  to  be  similar  in  kind  to  that 
experienced  when  the  senses  of  sight  and  touch  are  stimu- 
lated  too   rapidly   for  the   bestowal   of  attention   on   each 
stimulus,  and  yet  too  slowly  to  give  rise  to  central  sensory 
fusion.      A  flickering  light  has  a  similar  effect.      The  mind 
seeks,  as  it  were,  to  maintain  order  in  the  reception  of  the 
messages  of  sense,  to  give  to  each  sensation  its  due  recogni- 
tion, and  yet  to  subordinate  it  to  general  relationships  and 
conscious  sequence.      But  the  stimuli  come  on  the  border- 
line  between  what  may  be   grasped  and   what   may  not. 
Before  the  sensorium  has  had  time  to  give  full  effect  to  one 
stimulus  another  has  come  upon  it,  and  finds  it  partly  ready 
but  not  quite,  or,  from  the  physical  point  of  view,  the  sen- 
sory centre  has  not  had  time  to  recover  completely  from  the 
disintegrating  effect  of  one  shock  before  it  has  to  endure 
another.      Something  is  being  impressed  upon  the  receptive 
centres  which   tends  to  force  the  mind  from  the  path   in 
which   it   seeks  to   move,   and  which   is  itself  followed   by 
another   and   another  claimant   for  notice,   till  we  become 
irritated  at  the  disturbance  and  weary  of  the  repeated  dis- 


262  Physiology  of  the  Senses 

traction.  All  this,  of  course,  takes  place  in  a  semi-uncon- 
scious way,  since  it  is  not,  as  a  rule,  the  beat  in  the  sound 
or  the  flicker  in  the  light  to  which  we  wish  to  pay  attention  ; 
the  pure  musical  sound  with  which  the  beat  interferes,  or 
the  thing  seen,  now  clearly,  now  dimly,  in  the  changing  light, 
is  the  object  of  mental  effort.  Without  analysing  the  nature 
of  the  disturbing  element,  we  feel  that  it  is  there,  and  to 
this  must  in  the  main  be  attributed  the  disagreeable  effect 
produced. 

Yet  while  this  holds  true  of  long-prolonged  tones  roughened 
by  fast-repeated  beats,  it  must  be  remembered  that  in  ordi- 
nary orchestral  music  we  rarely  hear  notes  entirely  free  from 
beats.  While  the  various  notes  of  a  chord  struck  upon  a 
piano  may  be  of  such  pitch  as  not  to  generate  beats,  the 
overtones  of  these  interacting  on  one  another  most  prob- 
ably will.  Certain  chords,  no  doubt,  are  freer  from  such 
roughness,  and  it  is  no  uncommon  thing  to  heighten  the 
effect  of  a  pure  harmonious  note  by  causing  it  to  be  preceded 
by  a  discord.  Contrast  in  sound,  as  in  colour,  heightens 
the  effect  on  the  sensorium.  The  eye  fatigued  by  looking 
at  a  red  colour  will,  when  turned  to  a  green  surface,  see  it 
of  intenser  hue  ;  the  ear  has  a  keener  appreciation  of  pure 
harmony  when  the  harsh  note  has  ceased  to  jar. 

Noise. — When  auditory  stimuli  are  non-periodic  in  char- 
acter the  resultant  sensation  is  that  of  a  noise.  A  single 
variation  of  pressure  upon  the  tympanum  might  be  sufficient 
to  set  the  mechanism  of  hearing  in  action,  but  the  resultant 
sound  could  not  be  musical  in  character.  It  has  been  held 
by  some  that  two  impulses  exactly  alike,  and  the  one  quickly 
following  the  other,  may  give  rise  to  a  musical  sensation, 
but  the  probability  is  that  the  musical  effect  is  in  this 
instance  due  to  overtones,  and  to  such  a  sound  it  is  not 
possible  to  assign  a  definite  pitch.  The  ear  can  easily  distin- 
guish as  separate  noises  the  effect  upon  it  of  impulses  coming 


Sound  and  Hearing  263 

at  the  rate  of  less  than  1 6  per  second.  When  the  noise  is 
due  to  vibrations  coming  at  the  rate  of  more  than  about  16 
per  second,  there  is  a  certain  amount  of  fusion  in  sensation, 
and  the  noise  has  for  us  a  certain  pitch.  Where  there  is 
an  initial  shock,  as  in  a  thunder-peal,  with  echoing  and  re- 
echoing at  somewhat  prolonged  intervals,  we  have  a  deep, 
rumbling  sound  ;  if  the  vibrations  succeed  one  another  very 
quickly  we  have  sounds  or  noises  of  high  pitch,  which  we 
describe  as  crackling,  whistling,  rustling,  shrieking,  creak- 
ing, and  so  on.  The  wind  sweeping  through  a  forest  sets 
up  an  infinite  number  of  intermittent  variations  of  aerial 
pressure  as  it  sways  branches  and  leaves  to  and  fro,  and  a 
low  rustling  sound  is  heard ;  but  when  it  agitates  tense 
structures,  such  as  the  cordage  of  a  ship's  rigging  or  the 
strings  of  an  ^Eolian  harp,  the  sound  becomes  more  dis- 
tinctly musical,  and  especially  if  the  wind  blows  with  a  fairly 
constant  force.  The  harsh  nature  of  the  sound  educed  from 
a  violin  by  an  unskilled  performer  is  due  to  inequalities  of 
pressure  upon  the  strings  with  the  bow,  while  the  master 
hand,  by  maintaining  steady  continuous  pressure  for  longer 
or  shorter  intervals,  and  thus  eliminating  discordant  over- 
tones, will  draw  forth  pure  melodious  sounds. 

General  Mode  of  Action  of  the  Ear. — Having  con- 
sidered the  structure  of  the  ear  and  the  physical  nature  of 
sound,  we  have  next  to  see  how  the  one  is  adapted  to  the 
other,  how  the  ear  responds  to  auditory  stimuli.  Much 
may  be  learned  from  the  study  of  pure  physics  as  to  the 
beauty  of  the  mechanical  adaptations,  but  this  merely  brings 
us  to  the  threshold  of  sensation.  The  changes  in  the  audi- 
tory nerves  and  nerve  centres  which  accompany  or  give 
rise  to  the  sensation  of  sound  are  almost  entirely  unknown. 
Even  with  regard  to  the  mode  of  action  of  the  internal  ear 
there  is  still  much  uncertainty. 


264  Physiology  of  the  Senses 

The  external  ear^  we  have  seen,  acts  mainly  as  a  collector 
of  sound  waves,  and  the  external  meatus,  closed  internally 
by  the  drum-head,  helps,  like  von  Helmholtz's  resonators,  to 
increase  the  energy  with  which  the  membrane  is  agitated. 

The  middle  ear  is  so  constructed  as  to  diminish  as  little 
as  possible  the  power  of  the  aerial  vibrations  in  their  trans- 
mission to  the  sensory  terminals.  When  vibrations  pass 
directly  from  air  to  solids  or  liquids,  much  of  their  energy  is 
lost.  If  a  membrane  intervenes  between  the  air  and  a 
liquid,  the  energy  is  not  lost  to  so  great  an  extent.  There  is, 
therefore,  mechanical  advantage  in  the  separation  of  the  fluids 
of  the  internal  ear  from  the  air  by  the  membranes  closing  the 
round  and  oval  windows.  But  these  membranes  are  small 
of  size,  tense  in  texture,  and  in  apposition  upon  one  side 
with  fluid  in  an  enclosed  space.  They  have  thus  little 
amplitude  of  movement.  This  is  compensated  forl^y  the 
drum-head.  Being  larger  than  the  membrane  of  the  oval 
window,  and  having  air  upon  both  sides,  it  vibrates  freely, 
and  being  firmly  attached  to  the  tympanic  ring  and  tense  in 
the  greater  part  of  it,  its  vibrations  are  readily  transmitted 
to  the  attached  chain  of  bones,  and  by  them,  with  little  if 
any  loss  of  power,  to  the  foot  of  the  stirrup-bone  with  its 
membranous  attachment  to  the  circumference  of  the  oval 
window,  and  so  to  the  perilymph.  Nay,  there  may  be  an 
actual  gain  from  the  lever  action  of  the  chain  of  bones  and 
the  greater  size  of  the  drum-head  (p.  213).  The  chain  of 
bones,  working  freely  in  the  middle  ear,  gives,  as  we  have 
seen,  a  greater  amplitude  of  movement  than  would  be  avail- 
able if  the  internal  ear  were  simply  buried  deeply  in  the 
cranial  bones.  Still,  the  ligamentous  connection  of  the  bones 
with  the  membranes  and  the  walls  of  the  tympanum  hinders 
over-movement,  and  enables  them  to  act  as  dampers,  pre- 
venting unnecessary  oscillation  of  the  drum-head.  The 
tenseness  of  the  membrane  and,  consequently,  its  power  of 


Sound  and  Hearing  265 

responding  to  sounds  of  different  pitch  and  intensity  are 
likewise  regulated  by  the  intrinsic  muscles  of  the  middle 
ear,  and  more  especially  by  the  tensor  tymfiani  muscle, 
while  the  entrance  of  air  by  the  Eustachian  tube  maintains 
equality  of  atmospheric  pressure  upon  the  two  sides  of  the 
drum-head. 

Vibrations  then  may  reach  the  internal  ear  either  through 
its  osseous  walls  or  through  the  membranes  of  the  oval  and 
round  windows.  In  the  vestibule  and  semicircular  canals 
these  vibrations  are  further  transmitted  to  the  membranous 
labyrinth  through  the  perilymph,  for  the  connection  of  this 
part  of  the  auditory  sac,  with  its  surrounding  walls,  is  by  no 
means  so  close  as  in  the  case  of  the  cochlear  canal.  Through 
the  membranous  sac  the  vibrations  reach  the  endolymph,  and 
so  come  to  the  terminations  of  the  vestibular  portion  of  the 
auditory  nerve  in  the  maculce  of  the  utricle  and  saccule,  and 
in  the  cristce  of  the  ampullae  of  the  semicircular  canals. 
The  effect  maybe  enhanced  by  the  otoconia  (p.  227)  in  the 
endolymph,  and  by  the  rods  projecting  from  the  auditory 
epithelial  cells  ;  for,  as  has  been  pointed  out,  the  hand 
thrust  into  water  may  be  incapable  of  detecting  the  presence 
of  sound  waves  passing  through  the  water,  but  will  easily  do 
so  if  grasping  a  rod.  This  will  be  readily  understood  if  we 
consider  that  the  rod  will  act  as  a  lever,  and  so  increase  the 
effect  of  the  sound  waves  on  the  hand. 

That  the  auditory  hairs  do  actually  sway  to  and  fro  under 
the  influence  of  sonorous  vibrations  may  be  taken  as  proved, 
for  Hensen  has  seen  with  low  microscopic  powers  the  audi- 
tory hairs  of  Mysis  (the  opossum  shrimp)  vibrating  in 
response  to  the  notes  of  a  keyed  horn.  The  auditory  hair- 
cells  are  either  the  terminations  of  the  auditory  nerve  fibres, 
or  are  in  close  apposition  with  them,  and,  on  receipt  of  the 
vibrational  stimulus,  an  impulse  is  given  to  the  nerve  ;  but 
at  this  point  we  are  arrested,  for  we  do  not  know  whether 


266  Physiology  of  the  Senses 

or  not  the  nerve  current  corresponds  in  rate  of  intermission 
with  the  variation  of  pressure  due  to  sound,  whether  vibra- 
tions are  transmitted  along  the  nerve,  or  whether  we  have 
to  do  with  an  entire  change  of  physiological  phenomena  in 
the  development  of  the  nerve  current. 

In  the  case  of  the  cochlea,  the  vibrations  may  be  trans- 
mitted by  the  perilymph,  and  through  the  membrane  of 
Reissner  and  the  cochlear  endolymph,  or  through  the 
basilar  membrane  to  the  endings  of  the  cochlear  branch  of 
the  auditory  nerve  in  Corti's  organ,  or  sonorous  vibrations 
of  the  bones  of  the  skull  may,  through  the  medium  of  the 
spiral  osseous  lamina  and  Bowman's  spiral  ligament,  be 
directly  transmitted  to  the  basilar  membrane  and  its  super- 
jacent structures. 

From  noting  the  mode  of  termination  of  the  cochlear 
nerve  in  or  round  the  hair-cells  of  Corti's  organ,  and  from 
the  analogy  of  the  nerve-endings  in  hair-cells  in  the  case  of 
the  other  special  senses,  we  cannot  but  infer  that  the  hair- 
cells  in  the  organ  of  Corti  form  the  peripheral  sensory  ter- 
minals, while  the  rods  of  Corti  and  the  supporting  cells  of 
Deiter,  with  their  phalangeal  connections,  serve  mainly  to 
transmit  to  the  hair-cells  the  vibrations  set  up  in  the  basilar 
membrane. 

In  all  parts  of  the  fluid  of  the  internal  ear  changes  of 
pressure  due  to  movements  of  the  chain  of  bones  must  be 
experienced,  and  as  the  fluid  is  incompressible,  there  must 
be  an  outward  or  inward  movement  of  the  membrane  of  the 
round  window  corresponding  respectively  to  every  inward  or 
outward  movement  of  the  stapes.  The  question  therefore 
arises  :  Do  all  parts  of  the  internal  ear,  or  at  least,  do  all 
the  terminations  of  the  auditory  nerve,  respond  alike  to  the 
sound  ;  or  does  each  nerve-ending  have  a  special  duty  to 
perform,  have  a  special  response  to  a  special  element  of  the 
sound,  be  it  pitch,  intensity,  or  quality  ? 


Sound  and  Hearing  267 

The  semicircular  canals  i?i  relation  to  i7ioveme?its.  — Con- 
sidered merely  from  an  anatomical  point  of  view,  we  should 
expect  a  difference  in  function  corresponding  to  the  struc- 
tural differences  between  the  macula,  cristce,  and  organ  of 
Corli,  between  the  vestibular  and  cochlear  divisions  of  the 
auditory  nerve,  and  the  different  nerve  centres  to  which  they 
pass.  It  has  even  been  suggested  that  the  vestibular  nerve 
and  its  terminals  have  nothing  to  do  with  the  sense  of  hear- 
ing, but  have  to  do  with  the  sense  of  equilibrium  or  of  the 
position  of  the  head  in  space,  while  the  appreciation  of 
sound  is  relegated  to  the  cochlea  alone.  In  support  of  this 
view  it  has  been  pointed  out  that  the  semicircidar  canals,  with 
their  crista  acusticce,  may  be  destroyed  without  impairment 
of  the  sense  of  hearing.  At  the  same  time,  the  animal  be- 
gins to  perform  peculiar  movements  which  vary  according 
to  the  canal  destroyed.  If  either  of  the  canals  in  the 
vertical  plane  is  injured,  the  animal  rotates  its  head  round 
a  horizontal  axis  at  right  angles  to  the  plane  of  the  canal ; 
and,  if  the  horizontal  canal  be  injured,  rotation  takes  place 
round  a  vertical  axis. 

These  rotary  movements  being  similar  to  those  produced 
by  lesions  of  the  cerebellum,  and  being  apparently  asso- 
ciated with  a  disturbance  of  the  power  of  co-ordinating 
muscular  movement — a  power  which  depends  largely  upon 
the  sense  of  equilibrium — it  was  held  that  the  canals  have 
to  do  with  this  sense,  or,  as  suggested  by  Cyon  in  1872,  with 
sensation  as  to  the  position  of  the  head  in  space.  As  Crum 
Brown  has  shown,  the  canals  of  the  opposite  sides  of  the 
head  may  be  divided  into  three  sets  of  two  each  in  nearly 
identical  planes,  and  so  related  as  to  be  nearly  at  right 
angles  to  each  other.  When  the  head  is  moved  in  any 
direction,  the  fluid  in  the  canals  tends  to  move  in  the 
opposite  direction,  or  at  least  to  lag  behind  the  moving 
walls  of  the  canals,  just  as  when  we  rotate  a  vessel  contain- 


268  Physiology  of  the  Senses 

ing  water  the  inertia  of  the  water  prevents  its  moving  so 
quickly  as  the  vessel  at  first,  and  of  stopping  so  quickly 
when  once  set  in  motion.  As  the  volume  of  fluid  in  the 
canals  is  constant,  the  fluid  must,  however,  move  with  the 
head.  It  cannot  lag  behind,  but  there  will  be  variation  of 
pressure  due  to  inertia.  Thus,  according  to  Crum  Brown, 
"  in  each  of  the  three  pairs  of  canals  (right  and  left  hori- 
zontal, right  superior  and  left  posterior,  right  posterior  and 
left  superior)  the  two  canals  are  so  placed  that  when  rota- 
tion takes  place  about  the  axis  to  which  they  are  perpen- 
dicular, one  of  the  two  canals  moves  with  its  ampulla 
preceding  the  canal,  so  that  the  flow  or  tendency  to  flow 
(or  pressure)  is  from  ampulla  to  canal,  while  in  the  other  the 
ampulla  follows  the  canal,  and  the  flow  or  tendency  to  flow 
(or  pressure)  is  from  canal  to  ampulla.  If,  then,  we  sup- 
pose that  flow  from  ampulla  to  canal — or  adopting  Mach's 
view,  increase  of  pressure  in  the  ampulla — alone  stimulates 
the  hair-cells,  while  no-  effect  is  produced  by  flow  in  the 
opposite  direction — or  by  diminution  of  pressure  in  the 
ampulla — we  have  in  the  six  canals  a  mechanical  system 
capable  of  giving  us  an  accurate  notion  of  the  axis  about 
which  rotation  of  the  head  takes  place  and  of  the  sense  of 
rotation.'5 1  It  has  been  further  urged  that  the  macula  of 
the  utricle  and  saccule  have  to  do  respectively  with  the 
sense  of  movement  in  a  vertical  or  horizontal  straight  line, 
just  as  the  cushions  of  the  ampullae  respond  to  rotation. 

On  the  other  hand,  it  is  alleged  that  even  when  the 
auditory  nerve  is  destroyed  and  the  body  rotated,  a  sensa- 
tion of  rotation  comes  on  as  usual.  If  this  be  so,  the  canals 
cannot  be  essential  to  the  sense  of  position.  Again,  it  is 
held  that  we   cannot  dissociate  the  vestibular  nerve  from 

1  A.  Crum  Brown,  "  Cyon's  Researches  on  the  Ear,"  Nature, 
1878.  See  also  M'Kendrick's  Text- Book  of  Physiology,  vol.  ii. 
p.  694. 


Sound  and  Hearing  269 

auditory  sensation,  since  animals  which  can  undoubtedly 
hear  well  may  have  a  very  rudimentary  cochlea. 

On  the  whole,  it  seems  probable  that  the  vestibular 
nerve  can  respond  to  auditory  stimuli.  It  may  act  under 
the  stimulus  of  sound,  and  it  may  respond  to  differences  of 
intensity  of  sound,  but  can  it  lead  to  the  appreciation  of 
differences  in  the  pitch  of  sound  ?  To  this  question  we 
must  probably  give  a  negative  answer.  No  doubt,  in  the 
case  of  crustaceans,  Hensen  has  found  that  auditory  hairs 
of  different  lengths  respond  to  certain  notes  better  than  to 
others,  but  no  such  difference  of  length  in  the  auditory 
hairs  of  the  macules  or  crista  can  be  seen  in  the  human  ear, 
nor  any  difference  that  could  lead  us  to  imagine  that  one 
cell  should  respond  differently  from  another.  The  hairs  on 
the  hair-cells  of  Corti's  organ  are  still  shorter,  so  that  we 
cannot  conceive  that  they  have  any  differentiating  action 
as  regard  the  appreciation  of  pitch.  They  seem  to  act 
rather,  as  suggested  above,  as  minute  levers  by  means  of 
which  the  auditory  cells  are  rendered  sensitive  to  even 
the  slightest  movements  in  the  fluid  that  bathes  their  free 
surfaces. 

Analytic  Power  of  the  Ear. — Has  the  ear,  then,  any 
mechanism  which  enables  it  to  appreciate  differences  of 
pitch,  or  to  analyse  a  compound  tone  into  its  constituent 
partial  tones  ?  There  is  a  fusion  of  all  partial  series  of 
vibrations  in  the  air  of  the  external  ear.  The  tympanic 
membrane  vibrates  as  a  whole,  and  responds  to  the  com- 
pound summational  wave,  however  complex  its  form  may 
be — that  is  to  say,  however  quickly  it  changes,  and  propor- 
tionally in  extent  to  the  variations  of  atmospheric  pressure. 
With  the  drum-head  moves  the  chain  of  bones,  and  with  it 
again  the  perilymph  and  the  endolymph.  Yet,  in  the  sen- 
sorium,  we  can  appreciate  either  the  quality  of  the  complex 
tone,  or  we  can  attend  to  its  constituent  parts.      Wherein 


270  Physiology  of  the  Senses 

comes  the  power  of  analysis  ?  Is  it  the  case,  as  Ruther- 
ford holds,  that  the  hairs  of  all  the  auditory  cells  vibrate 
to  every  tone,  just  as  the  drum  of  the  ear  does,  and  that 
there  is  no  analysis  of  complex  vibrations  in  the  coch- 
lea or  elsewhere  in  the  peripheral  mechanism  of  the  ear ; 
that  the  hair- cells  transform  sound  vibrations  into  nerve 
vibrations,  similar  in  frequency  and  amplitude  to  the  sound 
vibrations  ;  that  simple  and  complex  vibrations  of  nerve 
molecules  arrive  in  the  sensory  cells  of  the  brain,  and  there 
produce  not  sound  again,  of  course,  but  the  sensation  of 
sound,  the  nature  of  which  depends,  not  upon  the  stimula- 
tion of  different  sensory  cells,  but  on  the  frequency,  ampli- 
tude, and  form  of  the  vibrations  coming  into  the  cells, 
probably  through  all  the  fibres  of  the  auditory  nerve  ? 1 

Upon  this  theory  the  whole  internal  ear  vibrates  in  unison 
with  the  drum-head,  and  the  auditory  nerve  in  unison  with 
both,  just  as  the  receiving  plate  of  a  telephone  moves  in 
unison  with  the  transmitting  plate.  Analysis  must  then  be 
a  mental  act  dependent  upon  the  powers  of  the  central  nerve 
cells,  but  how  it  is  to  be  exercised  we  are  not  informed. 

Or  does  the  power  of  analysis  lie  with  the  cochlea  ?  This 
is  the  theory  which  von  Helmholtz  first  stated  and  explained 
with  consummate  skill.  We  have  seen  (p.  255)  that  when 
a  compound  tone  is  sounded  before  a  piano  w'ith  uplifted 
dampers,  the  strings  of  the  piano  which  are  in  tune  with  the 
partial  tones  of  the  compound  tone  will  vibrate.  Similarly, 
von  Helmholtz  conceived  that  the  cochlea  has  the  power  of 
analysing  compound  tones  into  simple  pendular  vibrations, 
and  that  different  parts  of  the  cochlea  respond  each  to  the 
particular  partial  to  which  it  is  attuned.  At  first,  he  sup- 
posed the  rods  of  Corti's  organ  were  the  structures  which, 
varying  in  size  and  shape,  took  up  each  its  own  tone,  and, 

1  Rutherford,  "On  the  Sense  of  Hearing,"  The  Lancet,  January 
1387. 


Sound  and  Hearing  271 

by  striking  upon  or  otherwise  exciting  the  hair-cells  with 
which  they  were  connected  by  means  of  the  phalanges,  caused 
sensory  stimuli  to  be  sent  by  the  nerve  fibres  attached  to 
the  hair-cells  to  corresponding  nerve  cells  in  the  sensorium. 
He  did  not,  however,  suppose  that  the  nerve  current  re- 
sembled physically  in  any  way  the  vibration  which  roused 
the  auditory  cell.  The  resulting  sensation  was  simply  due 
to  the  specific  power  of  the  cell  in  the  brain,  to  give  rise  to 
a  sensation  of  a  sound  of  a  certain  pitch  when  stimulated 
by  its  proper  tone. 

Various  considerations,  however,  induced  him  to  modify 
his  theory.  In  the  first  place,  the  rods  of  Corti  vary  very 
little  in  form  and  size,  as  we  pass  from  the  base  to  the  apex 
of  the  cochlea.  Again,  there  are  only  about  3000  of  them 
altogether,  and  yet  we  can  distinguish  differences  of  pitch 
in  sounds  varying  in  their  number  of  vibrations  from  30  to 
40,000  per  second.  Further,  we  have  good  grounds  to 
believe  that  birds  can  distinguish  the  pitch  of  tones,  and 
yet  the  rods  of  Corti  are  entirely  absent  from  their  cochleas 
which  have  the  hair-cells  in  contact  with  the  basilar  mem- 
brane, and  are  very  rudimentary  in  other  respects.  For 
these  and  similar  reasons,  von  Helmholtz  supposed  that 
the  real  analysers,  in  respect  of  pitch,  are  the  fibrils  in  the 
outer  part  of  the  basilar  membrane,  and  that  the  rods  of 
Corti  simply  serve  to  pick  up  and  transmit  their  vibrations 
to  the  hair-cells.  This  view  is  supported  by  the  fact  that 
the  basilar  membrane  is  stretched  firmly  in  the  direction  of 
these  fibrils,  but  is  loose  in  the  direction  of  the  canal.  The 
fibres  are  easily  separated  from  one  another,  but  are  not 
readily  torn  across.  The  membrane  will  not  vibrate,  as  a 
whole,  like  one  in  which  the  tension  is  alike  in  all  directions, 
but  it  is  made  up  of  strings  or  fibres,  each  of  which  may 
vibrate  independently  of  the  other. 

There  are  about  "24,000  of  these  fibrils  in  the  basilar 


272  Physiology  of  the  Senses 

membrane — a  number  much  larger  than  that  of  the  rods  of 
Corti,  although  less  than  the  number  of  sounds  between 
which  we  can  make  a  distinction  of  pitch.  Von  Helmholtz 
supposed,  then,  that  these  fibrils,  varying  in  length  and 
possibly  in  tension,  may  respond  in  sympathetic  vibration 
each  to  its  proper  tone,  and  that  these  vibrations  are  trans- 
mitted to  the  hair-cells  by  their  supporting  structures.  If  a 
tone  falls  upon  the  ear  which  does  not  correspond  exactly 
in  vibrational  frequency  with  that  of  any  of  the  fibrils,  von 
Helmholtz  suggested  that  two  or  more  adjacent  fibrils  might 
respond  in  various  degrees,  that  being  strongest  which 
approximated  most  nearly  to  the  stimulus,  the  others  more 
feebly.  By  a  mental  combination  and  comparison  of  the 
different  stimuli  the  true  pitch  of  the  note  would  be  arrived 
at.  Thus  each  fibril  has,  according  to  him,  one  proper 
tone  to  which  it  answers  strongly,  while  to  all  others  it  is 
less  responsive.  Similarly,  in  the  case  of  the  stimulation 
of  the  auditory  hairs  of  My  sis,  it  was  found  that  different 
hairs  responded  strongly  to  different  tones.  One,  for 
example,  vibrated  strongly  to  cfiJL  and  d'§,  more  weakly  to 
g,  and  very  weakly  to  G.  Another  hair  answered  strongly 
to  a§  and  adjacent  tones,  more  weakly  to  d§  and  A$.  For 
some  tones,  then,  the  cerebral  cells  are  directly  tuned,  but 
not  for  others  ;  for  all  others  there  must  be  a  comparison 
of  several  tones  and  appreciation  of  pitch  through  the  means 
of  an  average.  As  von  Helmholtz  does  not  suppose  that  the 
nerve  current  in  any  way  corresponds  in  number  of  vibrations 
to  that  of  the  exciting  cause,  each  nerve  cell  depends  on  its 
own  inherent  power  of  response  in  giving  rise  to  a  sensation 
of  a  special  pitch.  But,  further,  it  has  been  computed  that 
there  are  only  about  15,000  hair-cells,  and  if  it  be  the  case 
that  each  of  these  is  connected  with  one  nerve  fibre  and  its 
special  brain  cell,  and  that  each  hair-cell  corresponds  only  to 
one  tone,  the  number  of  special  tones  to  be  directly  recognised 


Sound  and  Hearing  273 

in  the  brain  is  considerably  less  than  the  number  of  fibrils 
of  the  basilar  membrane  would  lead  us  to  expect.  On  the 
other  hand,  if  the  cell  may  respond  to  more  than  one  tone, 
and  give  rise  to  sensations  of  different  tones  in  the  sen- 
sorium,  we  must  have  some  difference  in  the  nerve  currents 
transmitted  at  different  times  from  periphery  to  centre  by 
the  same  nerve,  and  this  would  probably  correspond  to 
different  rates  of  vibration  of  the  basilar  fibrils. 

Now,  it  is  just  possible  that  there  may  be  a  greater 
power  of  response  in  the  basilar  membrane  to  sounds  of 
varying  pitch  than  von  Helmholtz  supposes.  If  at  any 
particular  moment  there  is  no  fibril  attuned  to  the  pitch  of 
the  incoming  sound,  it  may  be  that  the  tension  of  part  of  the 
membrane  may  be  varied  to  suit  the  exigencies  of  the  case. 
We  have  seen  that  Bowman's  ligament,  by  which  the  basilar 
membrane  is  attached  to  the  outer  wall,  contains  spindle 
cells  which  may  be  regarded  as  muscular,  and  by  the  con- 
traction of  which  the  pull  upon  the  fibrils  may  be  varied, 
and  their  tension  increased  or  diminished.  A  similar  result 
might  follow  a  change  in  the  amount  of  blood  circulating 
in  the  spiral  ligament,  giving  more  or  less  turgidity  to  this 
structure.  Thus  if  each  fibril  of  the  basilar  membrane  in  its 
normal  condition  of  length  and  tension  is  tuned  approxi- 
mately to  a  special  tone,  and  if  by  variation  of  its  length  or 
tension  it  may  be  rendered  responsive  to  tones  of  slightly 
higher  or  lower  pitch,  as  we  may  tune  a  violin  by  tightening 
or  slackening  the  strings,  we  have  in  the  ear  a  complete 
analysing  mechanism  for  the  pitch  of  all  musical  sounds. 
Such  an  hypothesis  renders  it  possible  likewise  that  we  may 
have  a  complete  series  of  tones  from  the  lowest  to  the 
highest,  melting  one  into  the  other  by  imperceptible  change — 
an  ear,  in  fact,  that  can  appreciate  the  pitch  of  any  possible 
tone  between  the  lowest  and  the  highest  limits,  a  capacity 
which  experience  shows  to  be  possible  in  the  human  ear, 

T 


274  Physiology  of  the  Senses 

and  that  directly  for  all  tones,  and  not  indirectly  for  some, 
as  von  Helmholtz  holds. 

If,  further,  it  is  the  case,  as  Rutherford  suggests,  that 
the  sensation  varies  in  the  central  cell  according  to  the  rate 
at  which  the  peripheral  end  of  the  nerve  fibre  or  the  hair-cell 
is  stimulated,  we  arrive  at  a  view  which  is  free  from  objec- 
tions that  may  be  urged  to  the  theories  both  of  Rutherford 
and  von  Helmholtz.  Rutherford's  theory  is  unsatisfactory 
in  so  far  as  it  entirely  disregards  the  elaborate  structure  and 
wonderful  complexity  of  the  cochlea,  deprives  the  ear  of  any 
analysing  power,  and  relegates  that  function  to  the  brain, 
among  whose  cells  we  can  find  nothing  in  any  way  suitable, 
from  a  morphological  point  of  view,  to  lead  to  a  perception  of 
variation  of  pitch.  The  physical  basis  for  analysis  must  be 
either  in  the  ear  or  the  brain  ;  but  if  all  parts  of  the  ear,  and 
all  the  fibres  of  the  auditory  nerve,  and  all  the  auditory  nerve 
cells,  respond  together  and  vibrate  alike,  we  have  no  such 
basis.  To  have  the  power  of  selecting  one  or  other  partial 
tone,  and  of  devoting  attention  to  it  °.lone  while  others 
are  still  affecting  the  sensory  mechanism,  it  seems  to  us  that 
there  must  be  several  structures  in  vibration  or  molecular 
change  at  different  rates.  If  the  auditory  centre  is  in  vibra- 
tion or  molecular  action  as  a  whole,  and  similarly  in  all 
its  parts,  it  is  impossible  to  understand  how  a  mere  effort  of 
will  can  enable  us  to  note  constituent  parts  of  a  complex 
tone.  We  can  pay  attention  to  one  or  other  partial  tone  in 
a  complex  sound,  just  as  we  can  fix  our  regard  upon  one 
part  of  the  field  of  vision  to  the  exclusion  of  all  the  rest,  but 
how  can  this  be  done  if  all  parts  of  the  auditory  centre  are 
affected  alike  ?  To  each  part  of  the  retina  there  is  a  cor- 
responding part  in  the  cortex  of  the  brain  ;  there  is  probably 
a  similar  relationship  between  different  parts  of  the  cochlea 
and  the  auditory  centre. 

On  the  other  hand,  the  main  objections  to  von  Helm- 


Sound  and  Hearing  275 

holtz's  theory  are  the  limited  number  of  structures  compared 
with  the  known  capacity  of  the  ear  and  the  supposition  that 
each  brain  cell  is  concerned  only  with  the  perception  of  one 
tone  in  different  degrees  of  power.  All  are  agreed  that  the 
cerebral  centres  can  appreciate  variations  in  strength  of 
stimulus.  In  all  the  special  senses  the  strength  of  the 
sensation  varies  with  the  strength  of  the  stimulus.  Now, 
this  does  not  necessarily  imply  in  regard  to  the  auditory 
nerve  that  the  actual  vibration  of  the  endolymph  is  trans- 
mitted as  a  vibration  that  might  be  seen  passing  along 
the  auditory  nerve  as  we  might  see  a  wave  of  vibration 
passing  along  a  tensely-stretched  rope  when  it  is  struck,  but 
it  does  imply  a  greater  molecular  movement  in  one  case 
than  in  another,  and  a  greater  or  less  effect  upon  the  proto- 
plasm of  the  receptive  nerve  centre.  There  may  be  no  real 
to-and-fro  vibration  of  the  nerve  corresponding  to  that  of  the 
internal  ear,  but  there  must  be  a  variation  in  the  nerve 
current  in  respect  of  amount  of  movement.  If  the  nerve 
cell  can  respond  to  variations  in  intensity,  there  is  no  greater 
difficulty  in  supposing  that  a  cell  whose  function  is  to  give 
rise  to  a  sensation  of  pitch  may  give  slightly  different  sensa- 
tions corresponding  to  slight  variations  in  the  rate  of  stimu- 
lation.1 If  it  be  urged  that  this  again  relegates  distinction 
of  pitch  to  the  brain,  and  that  we  might  as  well  suppose 
each  auditory  cell  to  have  the  power  of  discriminating 
between  all  degrees  of  pitch,  we  would  answer  that  the 
multiplication  of  centres,  each  having  slightly  different 
receptive  powers,  affords  an  anatomical  basis  for  the  simul- 
taneous reception  of  many  stimuli  differing  from  one  another 

1  See  also  the  remarks  on  the  modified  theory  of  colour  vision 
recently  propounded  by  von  Helmholtz  (p.  169).  This  distinctly 
favours  the  view  that  terminal  organs,  such  as  the  rods  and  cones  of 
the  eye  (and  why  not  the  delicate  mechanism  of  the  internal  ear?),  may 
respond  to  different  rates  of  vibration. 


276  Physiology  of  the  Senses 

only  it  may  be  in  the  matter  of  pitch,  while  by  allowing 
that  each  little  centre  may  give  slightly  different  pitch- 
sensation  with  variation  in  the  rate  of  stimulus  we  avoid  the 
difficulty  into  which  von  Helmholtz's  theory  plunges  us.  But, 
it  may  be  asked,  can  a  nerve  fibre  respond  in  this  way  to 
different  numbers  of  stimuli  per  second  ?  There  is  not  the 
least  doubt  that  it  can.  The  number  of  stimuli  sent  along 
a  nerve  to  a  muscle  may  be  largely  varied  with  varying 
effect  on  the  muscle  in  the  way  of  contraction.  In  the  case 
of  insects,  for  example,  the  wings  may  vibrate  as  often  as 
352  times  per  second  (Rutherford),  and  each  movement 
must  be  due  to  at  least  one  separate  nerve  impulse.  A 
nerve  removed  from  the  body  may  be  inserted  in  a  tele- 
phonic circuit,  and  it  will  conduct  the  electric  current  and 
transmit  the  delicate  variations  of  electrical  intensity  neces- 
sary for  telephonic  communication.  We  do  not  assert  that 
the  ordinary  nerve  current  is  electrical  in  character,  but  if 
the  nerve  can  transmit  variations  so  delicate  as  those  of  the 
telephone  must  be,  they  may  as  readily  be  deemed  capable 
of  responding  in  rate  to  their  normal  auditory  stimuli. 
Moreover,  it  must  be  borne  in  mind  that  the  sensation  of 
pitch  is  in  no  way  comparable  qualitatively  with  the  phy- 
sical changes  which  give  rise  to  it.  We  have  no  sensation 
of  each  individual  variation  in  the  stimulus.  The  sensorium 
fuses  the  impulses  so  as  to  give  rise  to  a  continuous  tone. 
And  again,  we  do  not,  as  a  rule,  note  the  partial  tones 
separately  and  respectively  :  indeed,  until  the  time  of  Tar- 
tini  they  were  not  known  to  exist,  and  until  the  time  of  von 
Helmholtz  were  deemed  of  small  importance.  Their  com- 
bination and  appreciation,  as  a  sound  of  determinate  quality, 
is  a  purely  mental  act,  combined,  that  is  to  say,  by  a 
mechanism  higher  than  and  different  from  the  initial  recep- 
tive auditory  centres.  It  is  only  when,  by  conscious  effort 
and  using  special  aids,  such  as  resonators,  we  pay  attention 


Sound  and  Hearing  277 

to  the  sensory  effect  that  we  note  the  constituent  parts. 
There  must  be  higher  mental  centres  in  which  fusion  occurs, 
or  a  unity  of  mind  in  which  a  synthesis  of  the  partial  sen- 
sations is  brought  about. 

The  Psychical  Elements  in  Auditory  Sensations. — 

When  the  auditory  centres  have  been  stimulated  and  the  sen- 
sation of  sound  receives  due  attention,  certain  mental  effects 
are  produced  which  are  superadded  to  the  simple  sensation 
of  sound.  We  judge,  for  example,  that  the  sound  has  been 
produced  outside  or  inside  of  the  body,  that  it  comes  in  a 
certain  direction  and  from  a  certain  distance,  or  we  may 
recognise  that  it  is  purely  of  a  subjective  character,  and 
exists  only  in  imagination.  In  arriving  at  a  decision  upon 
such  points  as  these  we  are  aided  by  the  other  senses 
and  by  knowledge  previously  acquired.  Thus,  when  we 
see  a  man  at  a  distance  from  us  lifting  a  gun  to  his  shoulder 
and  a  puff  of  smoke  issuing  from  the  muzzle,  we  know  from 
experience  that  we  will  shortly  hear  the  sound  of  the  detona- 
tion. We  infer  from  the  character  of  the  sound,  its  loudness, 
and  the  time  that  elapses  before  the  report  is  heard,  that 
it  comes  from  the  gun  and  from  no  other  source. 

Externality  of  Sound. — The  power  which  the  mind 
possesses  of  determining  whether  a  sound  originates  out- 
side or  inside  of  the  body  seems  to  be  in  large  measure 
dependent  upon  whether  the  sonorous  vibrations  are  com- 
municated to  the  ear  through  the  auditory  meatus,  the 
drum-head,  and  the  chain  of  bones,  or  directly  through  the 
bones  of  the  head.  We  mentally  project  the  source  of  the 
sound  outwards  when  the  vibrations  act  mainly  through  the 
meatus  on  the  tympanum,  but  if  the  sounding  body  is 
touching  the  head  we  may  have  the  impression  as  if  the 
sound  came  from  within  the  head.  Weber  has  pointed  out 
that  if  the  meatus  is  filled  with  water  the  idea  of  externality  is 


278  Physiology  of  the  Senses 

destroyed,  and  that  the  sound  seems  to  originate  in  the  head. 
Even  when  the  air  in  the  meatus  is  vibrating  freely  in  re- 
sponse to  sonorous  undulations,  if  the  body  emitting  the 
sound  touches  the  head,  the  idea  of  externality  may  dis- 
appear. Suppose  two  bodies  giving  out  exactly  similar 
sounds,  as  when  two  telephones,  connected  in  one  circuit,  are 
held  to  the  two  ears  and  made  to  respond  to  one  and  the 
same  sound.  If  the  telephone  to  the  right  side  be  tightly 
applied,  while  the  one  to  the  left  be  held  at  some  little  dis- 
tance from  the  ear,  the  sound  will  seem  to  originate  in  the 
right  side  of  the  head.  If  the  one  to  the  left  is  now  pressed 
closely  and  that  to  the  right  withdrawn  a  little,  the  sound  is 
heard  in  the  left  side  of  the  head,  but  if  both  instruments 
are  held  tightly  to  the  ears,  the  sound  seems  to  originate 
inside  of  the  head  and  towards  the  middle  line,  so  that 
it  will  be  described  by  one  observer  as  seeming  to  be  in 
the  mouth,  by  another  at  the  top  of  the  head,  and  by  a 
third  at  the  nape  of  the  neck.  Lastly,  by  slight  variations 
in  the  pressure  on  the  head  we  can  apparently  make  the 
sound  move  from  side  to  side  at  pleasure.  The  sound  of 
our  own  voice  is  heard  as  originating  within  the  head,  and 
certain  disorders  may  give  rise  to  sensations  of  sounds  re- 
ferred to  the  ears.  Thus  when  the  intracranial  circulation 
has  been  disturbed,  we  may  have  a  ringing  in  the  ears, 
or  may  hear  the  throbbing  of  the  pulse.  An  accumu- 
lation of  cerumen  or  wax  in  the  external  meatus  may  give 
rise  to  unpleasant  sounds  by  interfering  with  the  vibration 
of  the  drum-head.  Drugs,  such  as  quinine  or  salicin,  may 
cause  hissing  or  whistling  sounds,  or  even  a  sensation  of 
deafness,  by  interfering  with  the  nutrition  of  the  auditory 
centres,  and  the  insane  often  think  they  hear  voices  and 
sounds  on  account  of  disordered  and  abnormal  stimuli  in 
the  diseased  brain.  So  strong,  indeed,  is  the  power  of 
imagination  in  the  hallucinations  of  the  insane  that  nothing 


Sound  and  Hear i7ig  279 

will  persuade  them  that  the  voices  are  not  actually  coming 
from  an  external  source,  and  it  is  to  be  remembered  that 
the  sensations  are  at  least  real  to  them,  latent  impressions 
being  developed  or  obscure  memories  recalled  by  cerebral 
irritation.  Nay  more,  we  may  ourselves  under  certain  cir- 
cumstances by  an  effort  of  the  mind  give  rise  to  auditory 
hallucinations.  Much  pleasure  may  often  be  derived  from 
the  following  experiment.  If  when  in  bed,  lying  perfectly 
quiet,  and  with  no  sounds  breaking  the  stillness  of  the 
night,  we  think  the  music  of  a  song,  fixing  our  attention 
upon  the  music  but  not  humming  it,  we  may  sometimes 
seem  to  hear  it  being  sung  an  octave  higher  by  a  voice  ex- 
ternal to  ourselves — a  female  voice  apparently,  from  its 
delicacy,  tenuity,  and  high  pitch — and,  strange  to  say,  not 
exactly  synchronous  with  but  very  slightly  behind  our  own 
imaginary  singing.  When  the  hallucination  is  thoroughly 
established  and  we  resign  ourselves  completely  to  it,  the 
two  voices  may  seem  to  go  on  without  effort  on  our  part, 
and  we  ourselves  to  be  merely  passive  listeners.  The  least 
movement,  however,  or  wandering  of  the  thoughts  to 
another  subject,  immediately  dispels  the  illusion.  In  per- 
forming this  experiment,  it  is  most  probable  when  the  mind 
has  all  its  faculties  concentrated  upon  the  endeavour  to  hear 
the  faint  sound  that,  in  thinking  the  music,  we  actually  give 
rise  to  slight  variations  in  the  tension  of  the  auditory 
structures,  and  possibly  stimulate  the  auditory  centre 
through  the  auditory  nerve,  but  to  so  small  an  extent  as  to 
be  hardly  perceptible  to  the  senses,  or  it  may  be  that  with 
the  concentration  of  the  mind  upon  the  expected  sound  the 
nutrition  of  the  auditory  centre  is  involved.  It  might  even 
be  that  the  auditory  centre  is  stimulated  from  the  parts 
which  subserve  volition,  but  this  is  mere  conjecture,  for 
which  no  experimental  data  can  be  adduced  beyond  the 
well-established  fact  that  lower  centres  may  be  inhibited  or 


280  Physiology  of  the  Senses 

excited  by  influences  coming  from  higher  cerebral  centres. 
As  a  monarch  may  summon  his  ministers  and  invoke  their 
aid  or  dismiss  them  from  his  presence,  so  the  conscious 
mind  may  call  upon  the  senses  for  their  testimony,  or  may 
bid  them  be  silent,  and  the  obsequious  senses  do  some- 
times seem  to  give  that  answer  which  their  master  desires, 
although  they  have  no  true  warrant  for  so  doing. 

Direction  of  Sotmd. — We  have  seen  (p.  200)  that  the 
determination  of  the  direction  in  which  a  sound  has  come 
is  largely  due  to  the  greater  intensity  of  the  sound  in  one 
ear  than  in  the  other  owing  to  the  sound  waves  striking 
more  fully  and  directly  upon  one  ear  than  the  other.  If, 
however,  the  source  of  sound  is  in  a  plane  passing  forward 
through  the  middle  of  the  body  it  is  impossible  by  means 
of  this  alone  to  say  whether  the  sound  comes  from  behind 
or  in  front.  Judgment  as  to  direction  is  made  more 
accurate  by  moving  the  head  so  that  the  sound  falls  more 
intensely  now  on  one  side  now  on  the  other.  If  the  apex 
of  a  hollow  cone  or  the  ear-piece  of  an  ear-trumpet  be 
inserted  into  the  meatus  and  the  instrument  be  moved  for- 
wards and  backwards,  the  apparent  direction  of  the  sound 
may  be  largely  modified,  and  we  have  a  similar  change  if 
the  auricle  be  flattened  out  backwards  against  the  side  of 
the  head  or  brought  forward  with  the  hand. 

In  many  cases,  we  judge  the  sound  to  come  in  a  certain 
direction  from  knowing  where  it  probably  originates,  as 
when  we  hear  a  bell  rung  in  a  steeple  with  whose  position 
relatively  to  ourselves  we  are  acquainted.  It  is  easier  to 
judge  the  direction  of  noises  than  of  musical  sounds,  and 
that  mainly  because  there  is  a  slight  difference  in  the  quality 
of  the  sounds  coming  to  the  two  ears,  and  noises  having 
generally  more  partial  tones  than  musical  sounds,  the  differ- 
ence is  more  easily  noted  and  the  judgment  as  to  direction 
assisted. 


Sound  and  Hearing 


251 


Dista?ice  of  the  Source  of Sound. — The  ear  has  no  direct 
power  of  estimating  the  distance  from  which  a  sound 
comes,  since  it  only  becomes  cognisant  of  the  sound  when 
it  reaches  the  ear.  We  can  only  form  a  rough  estimate 
from  knowing  by  previous  experience  that  a  given  sound 
will  presumably  have  a  certain  intensity  when  produced 
at  a  certain  distance  from  us,  and  that,  other  things 
being  equal,  it  will  diminish  to  a  certain  extent  the  farther 
it  is  from  the  ear.  Experimentally,  it  has  been  proved 
that  when  sound  is  transmitted  through  a  fairly  homo- 
geneous medium,  as  through  air  or  water,  the  intensity  of 
the  sound  varies  inversely  as  the  square  of  the  distance. 
For  twice  the  distance,  the  intensity  will  be  one-fourth  ;  for 
three  times  the  distance,  one-ninth,  and  so  on.  But  if  we 
modify  the  conditions  for  the  transmission  of  sound,  our 
power  of  judgment  soon  fails  us.  If,  for  example,  when 
sitting  at  a  table  we  scratch  it  gently  with  the  finger-nail, 
the  arm  being  outstretched,  we  hear  a  sound  of  faint  inten- 
sity, the  distance  of  which  we  can  estimate  fairly  well ;  but 
if  the  ear  be  applied  to  the  table,  the  sound  seems  to  be 
made  at  the  ear,  its  intensity  not  having  been  materially 
diminished  by  transmission  through  the  wood.  Similarly, 
if  the  sound  is  transmitted  through  tubes,  the  law  of  diminu- 
tion of  intensity,  according  to  the  square  of  the  distance, 
does  not  apply,  and  we  hear  people  speaking  through  a  long 
tube,  as  from  top  to  basement  of  a  house,  as  if  they  were 
close  beside  us.  By  gradually  diminishing  the  intensity  of 
a  sound,  it  may  be  made  to  seem  to  come  from  a  consider- 
able distance  when  really  being  produced  close  at  hand. 
Thus,  when  the  operatic  chorus  leaves  the  stage,  and  dis- 
appears from  view  behind  the  scenes,  by  singing  more  and 
more  softly,  the  performers  can  convey  the  impression  that 
they  have  retired  to  a  great  distance.  So  the  art  of  the 
ventriloquist  lies  in  his  power  of  speaking  with  almost  no 


282  Physiology  of  the  Senses 

facial  movement,  of  changing  rapidly  the  strength  of  his 
voice  so  as  to  give  the  impression  of  varying  distance, 
and  of  conveying  by  gestures  that  the  sound  seems  to 
come  from  a  certain  spot,  whence  he  seems  to  hear  it 
coming,  just  as  we  do  ourselves.  A  slight  variation  in  the 
quality  of  a  sound  likewise  takes  place  as  it  recedes  from 
us,  certain  partial  tones  becoming  inaudible  sooner  than 
others  ;  this  too  may  help  our  judgment  as  to  distance. 

Memory  of  Sound. — It  is  sometimes  difficult  for  us  to 
judge  by  the  power  of  hearing  when  a  sound  has  ceased  to 
stimulate  the  ear.  When,  for  example,  a  bell  has  been 
ringing  for  some  time  and  then  stops,  the  sound  gradually 
dies  away,  and  it  is  almost  impossible  for  us  to  tell  the 
exact  moment  when  it  has  ceased.  It  may  seem  to  have 
died  away  entirely,  and  we  cease  to  strain  the  ear  to  catch 
its  faint  tones,  but  if  we  listen  again  we  seem  to  hear  it 
faintly.  This  may  be  due  to  different  causes.  It  may  be 
that  the  ear  has  become  fatigued  for  the  special  sound, 
and  that  the  momentary  withdrawal  of  the  attention  has 
rested  the  ear,  so  that  it  can  respond  to  tones  previously 
inaudible.  On  the  other  hand,  it  may  be  due  to  a  vivid 
form  of  memory.  We  cannot  doubt  that  there  is  some 
physical  change  in  the  auditory  centre  when  the  sensation 
of  sound  is  excited,  and  when  the  centre  has  once  acted  in 
a  particular  way  it  does  so  more  easily  when  similar  circum- 
stances again  arise,  or  even  as  the  result  of  a  mental  effort. 
Sometimes  it  may  require  repeated  attempts  before  we  are 
able  to  recollect  a  sound,  as,  when  after  hearing  a  new  song, 
we  fail  for  a  day  or  so  to  remember  the  music  of  it,  but 
gradually  note  by  note,  and  line  by  line,  it  returns,  often 
without  conscious  effort,  until  we  are  able  to  piece  it  all 
together  again,  more  or  less  correctly,  according  to  acute- 
ness  of  ear  and  receptivity  for  musical  impressions. 

Mental  Receptivity  for  Sound. — This  is  a  power  which 


Sound  and  Hearing  283 

varies  much  with  the  state  of  the  mind  and  the  nature  of 
our  environment.  As  a  rule,  we  pay  no  attention  to,  and 
do  not  consciously  hear,  such  customary  sounds  as  the  tick- 
ing of  a  clock,  the  noise  of  street  traffic,  and  the  like, 
although  they  must  be  constantly  acting  upon  the  ear.  They, 
indeed,  constitute  for  us  our  basis  of  silence,  so  to  speak,  for 
if  the  clock  should  stop,  or  if  we  pass  to  the  solitude  of  the 
country,  we  seem  to  hear  the  silence  which  ensues.  Again, 
just  as  some  people  are  colour  blind,  so  others  may  be  deaf 
to  the  pitch  of  sounds.  Some  ears  are  adapted  only  for 
sounds  of  comparatively  low  pitch,  others  for  high  pitch  ; 
they  are  deaf  to  all  others.  If  we  take  the  lowest  limit  for 
pitch  at  16  vibrations  per  second,  and  the  highest  at  about 
40,000,  we  have  in  all  a  range  of  about  1 1  octaves.  The 
ear  has  thus  a  much  wider  range  for  pitch  than  the  eye  for 
colour,  for  it  will  be  remembered  that  the  lowest  red  rays  of 
the  spectrum  have  a  vibrational  frequency  of  435  millions 
of  millions  per  second,  while  those  of  the  ultra  violet  are 
about  764  millions  of  millions — that  is  to  say,  less  than  twice 
the  number  at  the  lower  end  of  the  spectrum,  or  less  than 
one  complete  octave. 

But  the  power  of  distinguishing  tones  of  varying  pitch 
is,  with  some,  so  slight  that  they  are  quite  unable  to  distin- 
guish one  tune  from  another,  and  others  who  can  recognise 
the  difference  are  unable  to  sing  more  than  one  or  two 
notes  of  different  pitch. 

Binaural  Audition. — Some  persons  have  been  found 
who  seemed  to  have  the  two  ears  differently  tuned,  so  that 
the  same  sound  seemed  to  be  of  higher  pitch  to  one  ear 
than  to  the  other.  Under  normal  conditions,  although 
from  the  position  and  shape  of  the  ears  the  sound  waves 
which  fall  upon  the  drum -head  cannot  be  exactly  the 
same  in  form  nor  in  time  of  excitation,  yet  the  resultant 
sensations  in  the  auditory  centre  are  mentally  united,  and 


284  Physiology  of  the  Senses 

we  hear  one  sound,  not  two.  This  is  mainly  to  be  accounted 
for  by  the  fact  that  the  sensation  lasts  for  a  short  time 
after  cessation  of  the  stimulus,  and  the  two  sounds  are  so 
slightly  separate  in  time  as  to  blend  readily  with  one 
another.  Inasmuch  as  the  two  ears  enable  us  to  a  cer- 
tain extent  to  judge  the  distance  of  the  sounding  body, 
binaural  audition  is,  in  a  way,  comparable  to  binocular 
vision,  which  assists  in  the  perception  of  solidity  or  distance 
in  space. 


THE   PHYSIOLOGICAL  CONDITIONS   OF 
SENSATION 

In  the  preceding  sections  we  have  given,  in  the  first  place, 
a  general  view  of  the  mode  of  action  of  the  nervous  system, 
and  then  we  have  described  each  of  the  five  senses  in 
detail.  We  have  seen  that  external  agents,  such  as  light 
or  sound,  act  on  special  terminal  organs,  and  that  from 
these,  nervous  impulses  are  carried  by  the  nerves  of  sense 
to  the  central  nervous  organs.  In  these  central  nervous 
organs  molecular  changes  occur,  which  are  related  in  some 
way  to  conscious  states  or  sensations,  and  we  then  refer 
these  sensations  to  the  outer  world,  and  to  the  agent  which 
we  believe  to  be  their  primary  exciting  cause.  Further,  we 
know  that  these  sensations  may  give  rise  either  to  voluntary 
or  involuntary  movements,  and  that  they  may  influence 
many  organs  of  the  body,  causing,  for  example,  the 
voluntary  movement,  the  involuntary  start,  the  blush  of 
modesty,  or  the  pallor  of  fear,  the  more  rapid  action  of  the 
heart,  or  the  quickening  or  slowing  of  respiration.  The 
functions  of  the  central  nervous  organs  and  of  the  organs 
of  sense  are  so  closely  related  as  to  make  it  no  easy 
matter  to  form  a  conception  of  the  system  working  as 
a  whole.  The  progress  of  discovery  naturally  tends  to 
differentiation,  and  to  attaching  undue  importance  to  one 
organ  as  compared  with  others,  so  that  we  are  in  danger 


286  Physiology  of  the  Senses 

of  losing  sight  of  the  solidarity  of  the  whole  nervous 
system. 

During  the  profound  unconsciousness  of  coma,  or  of  deep 
sleep,  the  mind  is  at  rest.  There  are  no  thoughts  and  no 
interpretation  of  messages  from  the  sense  organs.  The 
higher  centres  of  the  brain  are  inactive,  but  lower  centres, 
such  as  those  governing  the  circulatory  and  respiratory 
mechanisms,  may  still  be  active,  the  heart  continues  to 
beat,  and  an  onlooker  sees  the  movements  of  respiration. 
During  the  waking  and  conscious  state,  however,  the  higher 
centres  are  active.  They  are  not  only  the  seat  of  molecular 
phenomena  related  to  the  conscious  state,  giving  rise  to  the 
revivications  of  memory,  the  play  of  ideas,  the  rise  of  desires 
and  impulses,  and  efforts  of  volition,  but  they  now  are 
momentarily  receiving  messages  from  the  various  sense 
organs.  These  messages  affect  the  higher  centres  them- 
selves, and,  through  them,  lower  centres  and  the  body 
generally.  Probably  every  nervous  action,  however  deli- 
cate and  evanescent,  affects  more  or  less  the  entire  system, 
and  thus,  in  addition  to  the  impulses  coming  from  the 
various  organs  of  sense,  there  may  be  an  undercurrent 
streaming  into  and  out  of  the  nerve-centres.  This  under- 
current may  never  give  rise  to  distinctly  conscious  states, 
but,  along  with  numerous  interactions  in  the  centres  them- 
selves, it  contributes  to,  and  partly  accounts  for,  the  appa- 
rent continuity  of  conscious  experience. 

No  one  doubts  that  consciousness  has  a  material  sub- 
stratum, but  the  problem  of  the  relation  between  the 
mental  state  and  the  molecular  movements  in  nervous 
matter  is  as  far  from  solution  as  in  the  days  when  little 
was  known  of  the  physiology  of  the  nervous  system.  Con- 
sciousness has  been  driven  step  by  step  upwards  until  it 
now  takes  refuge  in  a  few  thousand  nerve-cells  in  a  portion 
of  the  gray  matter  in  the  cortex  of  the  brain,  or  it  may  be 


Physiological  Conditions  of  Sensation        287 

in  the  dense  network  of  fine  fibrils  that  abounds  in  gray- 
matter.  The  ancients  believed  that  the  body  participated 
in  the  feelings  of  the  mind,  and  that  the  heart,  liver,  and 
reins  (kidneys)  were  connected  with  the  emotions,  a  view 
quite  consistent  with  the  familiar  experience  that  these 
organs  are  often  influenced  by  such  mental  states.  As 
science  advanced,  consciousness  was  relegated  to  the  brain, 
first  to  the  medulla,  and  lastly  to  the  cortex.  But  sup- 
posing we  were  able  to  understand  all  the  phenomena — 
chemical,  physical,  physiological  —  of  this  intricate  gan- 
glionic mechanism,  we  would  be  no  nearer  a  solution  of  the 
problem  of  the  connection  between  the  objective  and  sub- 
jective aspects  of  the  phenomena.  It  is  no  solution  to 
resolve  a  statement  of  the  phenomena  into  mental  terms 
or  expressions,  and  to  be  content  with  an  exclusively 
idealistic  theory  of  cognition.  Nor  is  it  more  satisfactory 
to  translate  all  the  phenomena  of  mind  into  terms  describ- 
ing physical  conditions,  as  is  done  by  those  who  support  a 
purely  materialistic  hypothesis.  A  philosophy  that  recog- 
nises both  sets  of  phenomena,  mutually  adjusted  and  ever 
interacting,  recognises  the  facts  of  the  case,  and  does  not 
delude  the  mind  by  offering  a  solution  which  is  in  reality 
no  solution  at  all.  The  difficulty  is  somewhat  lessened  if 
we  assume  that  behind  all  physical  and  mental  phenomena 
there  is  a  metaphysical  essence,  conscious  or  unconscious, 
and  that  the  phenomena  we  term  physical  and  mental  are 
only  different  sides  of  the  same  thing.  Such  an  essence 
can  never  be  known  to  science,  and  the  discussion  of  the 
possibility  of  its  existence  and  of  its  properties  belongs  to 
the  province  of  philosophy.1 

Apart  from  the  ultimate  question,  however,  there  is  the 
important  one  whether  physiologists  are  right  in  relegating 
consciousness  entirely  to  the  gray  matter  of  the  brain.    The 
1  Von  Hartmann,  Philosophy  of  the  Unconscious,  especially  vol.  iii. 


288  Physiology  of  the  Senses 

facts  of  comparative  physiology  are  against  a  view  so  exclu- 
sive, because  we  cannot  deny  consciousness  to  many  animals 
having  rudimentary  nervous   systems,  or  none  at  all.     As 
already  said,  research  in  anatomy  and  physiology,  and  the 
observation  of  disease,  have  obliged  physiologists  to  adopt 
the  view  that  the  brain  is  the  seat  of  sensation,  or,  in  other 
words,  of  consciousness.    This  is  no  doubt  true  in  the  sense 
that  it  receives  all  those  nervous  impulses  that  result  in  con- 
sciousness, but  parts  acted  on  by  external  physical  agents 
(like   the   retina)   and  the   parts   transmitting  the  nervous 
impulse  (like  the  optic  nerve)  are,  in  a  sense,  as  much  con- 
cerned in  the  production  of  conscious  states  as  the  brain 
itself.     This  view  of  the  matter  was  urged  by  Cleland  in 
1870,1  and  is  consistent  with  the  facts  of  nervous  physio- 
logy.     It  presents  fewer  difficulties  than  the  one  generally 
held  which  drives  consciousness   into  the  recesses  of  the 
nerve-cells  in  the  cortex  of  the  cerebral  hemispheres.      It 
keeps   clear   of  the  prevailing  error   in   the   philosophy  of 
modern  physiology,  that  of  regarding  the  body,  and  even 
the  nervous  system,   as  a  vast  collection  of  almost   inde- 
pendent organs,  losing  sight  of  community  of  function  and 
interdependence  of  parts.     At  the  same  time  it  must  be 
admitted   that  it  approaches  no  nearer  a  final  solution  of 
the  problem  of  the  origin  of  consciousness  ;  it  only  states 
the  conditions  of  consciousness  with  greater  precision. 

Let  us  now  approach  the  question  from  another  point  of 
view.  The  simplest  structural  nervous  unit  is  a  Cell, 
which  we  may  call  A,  with  a  fibre  passing  to  it  from  a 
specialised  cell,  B,  on  the  surface  of  the  body,  and  another 
fibre  passing  from  it  to  a  contractile  cell,  C.  A  stimulus 
applied  to  B  causes  molecular  changes  in  it,  which  result  in 
the  transmission  of  an  impulse  to  A,  in  which  molecular 
changes  again  occur,  resulting  in  the  transmission  of  an 
1  Cleland  on  Evolution,  Expression,  and  Sensation,  1870.  \ 


Physiological  Conditions  of  Sensation        289 

impulse  to  C.  This  is  the  simplest  form  of  a  so-called 
reflex  mechanism.  Suppose  the  same  kind  and  degree  of 
stimulus  be  applied  to  A  many  thousand  times  in  succes- 
sion, and  repeated  not  only  in  an  individual,  but  in  a  line  of 
individuals  genealogically  connected  as  parent  and  offspring, 
we  can  imagine  that  its  molecular  structure  will  become  so 
modified  that  it  will  gradually  become  more  and  more 
responsive  to  stimuli  of  this  kind,  the  simple  mechanism 
having  become  attuned  to  the  movements  of  the  outer 
world.  Here,  then,  we  have  a  molecular  condition  associated 
with  the  dawn  of  consciousness,  and  the  attuned  condition 
of  the  structure  may  be  regarded  as  the  beginning  of 
memory.  No  doubt  it  is  impossible  here,  just  as  in  dealing 
with  a  complex  brain,  to  form  any  conception  of  the  genesis 
of  consciousness.  It  evidently  cannot  be  the  result,  in  any 
physical  sense,  of  the  molecular  changes  in  the  cell,  because 
even  although  we  were  cognisant  of  all  the  molecular  changes 
we  could  not  detect  a  conscious  state.  So  far  as  an  out- 
sider is  concerned,  the  conscious  state  of  the  cell  can  only 
be  recognised  by  some  outward  manifestation  in  the  form  of 
movement,  and  it  is  conceivable  that  the  cell  might  be 
conscious,  and  yet  not  make  any  movement.  Suppose  a,  b, 
c,  d,  e,  etc.,  to  represent  links  in  the  chain  of  physical  phe- 
nomena between  the  irritation  of  the  cell  B  and  the  move- 
ment of  0,  and  that  consciousness  is  an  attribute  of  A, 
which  we  may  denominate  x,  it  will  be  impossible  to  find  a 
place  for  x  in  the  chain,  in  the  same  sense  as  the  movement 
of  C  is  the  last  link  of  the  chain.  It  cannot  come  in  be- 
tween a  and  b,  as  a  is  the  physical  antecedent  of  b,  nor,  for 
a  similar  reason,  between  b  and  c,  nor  between  c  and  d, 
d  and  e,  etc.  The  condition  x  is  therefore  outside  the 
physical  chain  ;  and  yet  it  is  related  to  it  so  intimately  as 
to  lead  to  the  illusion  that  x  forms  one  of  the  links. 
This   appears   to   prove   that   consciousness,  x,   is   outside 

U 


290  Physiology  of  the  Senses 

any  chain  of  related  physical  phenomena  conceivable  in  the 
simplest  nervous  mechanism. 

Nor  do  we  get  any  farther  towards  clearing  up  the 
mystery  if  we  suppose,  as  some  have  done,  that  even  dead 
matter  has  in  some  way  associated  with  it  units  of  con- 
sciousness,1 because  it  is  equally  impossible  in  this  case  to 
understand  the  nexus  between  the  material  particles  and 
consciousness.  The  condition  of  the  conscious  state  may 
therefore  be  represented  by  two  parallel  curves  infinitely 
close  together,  the  one  representing  the  chain  of  physical 
phenomena,  linked  together  as  cause  and  effect,  and  the 
other  the  chain  of  conscious  states.  Any  variation  in  the 
one  coincides  with  a  variation  in  the  other,  but  no  explana- 
tion can  be  given  as  to  how  the  one  influences  the  other. 
To  assert  that  one  is  the  cause  of  the  other  is  simply  to 
beg  the  question.  If  we  say  that  the  chain  of  physical 
phenomena  is  the  cause  of  the  conscious  states,  in  the 
same  sense  as  the  physical  phenomena  in  a  cell  of  the 
liver  is  the  cause  of  the  secretion  of  bile,  we  introduce  into 
the  chain  an  immaterial  something,  and  break  the  physical 
continuity  of  the  various  links  ;  and,  on  the  other  hand,  if 
we  try  to  escape  the  difficulty  by  translating  the  physical 
links  themselves  into  states  of  consciousness,  and  deny  any 
knowledge  of  the  physical  substratum,  we  are  deceived  by 
words  and  reach  no  solution. 

Again,  to  regard  consciousness  as  a  mode  of  e?iergy  is 
unsatisfactory.  Energy,  in  the  physical  sense,  is  nothing 
more  than  the  power  any  material  system  has  of  doing 
work,  owing  to  the  relative  position  of  its  component  parts. 
If  the  relative  position  of  these  parts  be  altered,  the  distri- 
bution of  energy  in  the  system  will  also  be  altered.  It 
follows  from  this  that  energy  may  be  manifested  by  various 
kinds  of  movements — heat,  light,  gravitation,  etc. — and 
1  W.  K.  Clifford,  Lectures  and  Essays,  vol.  ii.  p.  31. 


Physiological  Conditions  of  Sensation        291 

one  form  of  energy  may  be  resolved  into  another.  But 
when  motion  produces  heat,  there  is  a  quantitative  con- 
version of  energy  from  motion  to  heat,  which  is,  in  turn, 
another  mode 'of  motion.  If  we  now  assume  molecular 
changes  to  be  the  cause  of  consciousness,  these  molecular 
changes  also  produce  heat,  molecular  movements  associated 
with  chemical  action,  and  perhaps  movements  on  a  larger 
scale  ;  but  the  sum  of  these  resultant  forms  of  energy  is 
equal  to  the  energy  at  first  existent  in  the  physical  system, 
which  we  assume  to  be  also  the  seat  of  consciousness.  Con- 
sequently consciousness  does  not  come  into  the  dynamical 
chain.  It  cannot  be  measured ;  it  cannot  be  derived  from 
the  physical  energies,  nor  can  it  be  resolved  into  them.  It  is 
outside  the  chain.  Movements  of  matter,  therefore,  cannot 
be  resolved  into  consciousness,  or,  in  other  words,  conscious- 
ness is  not  a  form  of  energy. 

We  are  thus  face  to  face  with  an  insoluble  problem,  even 
when  we  discuss  it  in  its  simplest  form,  and  it  becomes 
infinitely  more  complicated  when  we  consider  the  manifold 
phases  of  consciousness  connected  with  the  mechanism  of 
the  brain.  If,  however,  we  begin  with  the  structural  unit 
of  a  simple  reflex  mechanism,  along  with  its  associated 
conscious  state,  we  find  that  the  complex  functions  of  the 
fully  -  developed  brain  are  aggregations  of  the  simple 
mechanism  we  have  considered,  and  that  what  we  term 
consciousness  is  a  condition  which  is  the  sum  of  the 
conscious  states  of  the  individual  nerve  cells,  or  aggrega- 
tions of  nervous  matter,  constituting  the  brain.  We  can 
form  no  conception  of  the  nature  of  the  consciousness  of 
a  nerve  cell  any  more  than  we  can  of  the  consciousness  of 
a  sea -anemone  or  of  a  worm  ;  but  we  must  assume  the 
existence  of  consciousness  in  a  nerve  cell,  otherwise  it  is 
impossible  to  understand  how  consciousness  is  associated 
with  an  aggregation  of  such  cells   in   a  brain.      To  deny 


292  Physiology  of  the  Senses 

consciousness  to  such  a  cell  would  be  equivalent  to  deny- 
ing consciousness  to  the  brain,  which  would  be  absurd. 
Whilst,  therefore,  we  give  up  the  explanation  of  the  genesis 
of  consciousness  as  an  insoluble  problem,  it  is  possible  to 
gain  some  insight  into  the  general  mode  of  action  of  brain 
as  the  recipient  of  sensory  impressions. 

Suppose,  for  example,  we  irritate  the  skin  of  the  sole  of 
the  foot,  an  impulse  is  carried  by  nerves  to  cells  in  the 
posterior  horns  of  gray  matter  in  the  spinal  cord  (see  Fig.  7, 
p.  16),  in  which  molecular  processes  are  excited.  From 
these,  impulses  are  carried  by  fibres  in  the  cord  to  cells  in 
the  anterior  horn ;  in  which,  again,  molecular  processes 
occur,  resulting  in  the  transmission  of  nervous  impulses 
along  motor  nerves  to  the  muscles  of  the  limb,  and  the 
limb  will  be  drawn  away  by  a  sudden  contraction  of  the 
muscles.  This  is  a  reflex  movement,  not  in  obedience  to 
a  volitional  impulse,  not  associated  with  consciousness  in  the 
usual  sense  of  the  term  (as  implying  activity  of  the  brain), 
but,  from  the  arguments  already  led,  we  may  assume 
that  these  molecular  changes  in  the  cells  of  the  cord  are 
associated  with  a  lower  mode  of  consciousness,  such  as 
presumably  exists  in  animals  having  a  nervous  system  of 
this  simple  type.  But  the  cells  in  the.  gray  matter  of  the 
cord  are  connected  with  cells  in  the  masses  of  gray  matter 
in  the  upper  centres,  and,  in  particular,  we  have  reason  to 
believe  that  each  unit  area  of  sensitive  surface  of  the  body 
has  a  corresponding  unit  area  in  the  cerebral  cortex,  that  is 
to  say,  from  each  unit  area  (the  size  of  which  varies  much  in 
the  different  sense  organs,  from  a  minute  area  of  retina  to  a 
much  larger  area  of  skin  surface)  nerve  filaments  pass  which 
carry  impressions  to  a  corresponding  unit  area  in  the  cortex 
(see  remarks  on  the  tactile  field,  p.  60,  and  on  the  visual 
field,  p.  30  and  p.  176).  This  does  not  mean  that  individual 
nerve  fibres   necessarily   pass   from   unit    area   of   sensory 


Physiological  Conditions  of  Sensation        293 

surface  to  unit  area  of  cortex,  but  that  impressions  are  so 
related.  If  so,  the  irritation  of  the  skin  of  the  foot,  in  the 
experiment  we  are  considering,  may  cause  impressions  to 
pass,  not  merely  to  the  cord,  but  also  to  the  higher  centres 
in  the  brain,  and  the  result  may  be  a  feeling  of  pain.  This 
may  be  also  explained  by  supposing  that  the  reflex  centre 
in  the  cord  is  intimately  connected  by  fibres  with  the 
conscious  centres  in  the  cortex,  a  supposition  strongly 
supported  by  ■  the  increasing  mass  of  evidence  as  to  the 
paths  of  transmission  between  the  cord  and  the  brain.  The 
sensation  of  pain  must  be  associated  with  molecular  changes 
in  the  cells  of  the  cortex,  and,  as  a  rule,  these  changes 
cause,  by  a  kind  of  irradiation,  the  transmission  of  impulses 
outwards  to  other  nerve  centres,  which  in  turn  call  forth 
various  more  or  less  complicated  movements.  Thus,  for 
example,  they  may  be  carried  to  the  cells  in  the  gray 
matter  of  the  medulla,  which  is  the  origin  of  the  nerves 
governing  the  movements  involved  in  crying,  in  the  ex- 
pression of  pain  by  the  muscles  of  the  face,  or  they  may 
reach  the  cells  in  the  gray  matter  of  the  cord,  calling  forth 
the  movements  of  the  limb  requisite  for  drawing  the  limb 
away  from  the  irritation,  or  for  defending  it  from  further 
attack.  Again,  the  irritation  may  call  forth  involuntary 
exclamations,  in  the  form  of  words,  expressive  of  pain,  and 
in  this  case  the  centre  for  articulate  speech  has  been  in- 
volved. Impressions  may  also  be  carried  from  the  sensory 
centre  in  the  cortex  to  the  parts  of  the  brain  concerned  in 
volition,  and  the  reflex  and  involuntary  movements  we  have 
considered  will  be  added  to,  or  supplanted  by,  direct 
voluntary  movements.  Even  voluntary  movements,  how- 
ever, are  essentially  reflex  in  character,  inasmuch  as  they 
are  called  forth  by  stimulations  which  have  been  applied  to 
a  sensory  surface  either  immediately  before  the  voluntary 
act,   or  which  have  been   applied,  it  may  be,  long  before. 


294  Physiology  of  the  Senses 

In  the  latter  case,  the  effects  of  the  stimulation  still  remain 
in  certain  groups  of  nerve  cells,  as  a  kind  of  memory,  so 
that  when  they  are  roused  into  activity,  the  voluntary  act 
will  follow,  as  it  probably  did  on  the  first  occasion  when 
the  stimulus  was-  applied.  Finally,  the  irritating  body  may 
be  seen,  and  the  effects  of  the  image  formed  optically  on 
the  retina  are  carried  by  the  optic  nerve  to  the  corpora 
quadrigemina,  and  from  these  to  the  visual  centres  in  the 
cortex.  Again,  a  memory  of  this  impression  may  remain, 
and  may  be  called  into  action  by  nervous  influences  coming 
from  other  parts  of  the  brain,  so  that  a  vzsioji  of  the  irritat- 
ing body  may  afterwards  arise  into  consciousness,  so 
vividly  as  to  call  forth  movements  similar  in  character, 
although,  probably,  not  so  intense,  as  those  which  occurred 
in  the  first  instance.  This  revivication  of  old  impressions 
is  most  likely  to  occur  when  the  upper  centres  are  some- 
what in  abeyance,  as  in  the  phenomena  of  hypnotism  and 
somnambulism. 

Sensory  impressions,  however,  are  not  only  carried  to 
the  cerebral  cortex,  there  awakening  consciousness,  but 
they  are  also  conveyed,  and  many  of  them  in  the  first 
instance,  to  the  cerebellum,  and  in  this  organ  they  set  in 
action  the  physiological  mechanism  that  results  in  co- 
ordinated movements.  It  is  not  improbable  that  the 
sensory  areas  of  the  body  have  corresponding  areas  in  the 
gray  matter  on  the  surface  of  the  cerebellar  convolutions. 
Thus  the  cerebellum  is  the  organ  that  gives  a  rhythmic 
character  to  certain  movements  of  the  body,  as  those  of 
walking,  flying,  swimming,  etc.,  and  probably  it  is  only  when 
these  movements  become  associated  with  sensation,  or  are 
voluntary,  that  the  centres  in  the  cerebral  cortex  come  into 
play. 

Again,  if  an  external  object  acts  at  the  same  time  on 
different  organs  of  sense,  as  when  we  hold  a  rose  in  the 


Physiological  Conditions  of  Sensation       295 

hand,  admire  its  colour,  and  enjoy  its  delicious  perfume, 
the  various  sensations  thus  related  to  molecular  movements 
in  different  parts  of  the  cortex  are  combined  by  the  action 
of  the  numerous  fibres  passing  from  centre  to  centre,  and 
the  result  is  a  conscious  perception  of  the  thing  as  a  whole. 
These  fibres  may  be  called  fibres  of  association,  because 
they  combine  impressions  that  have  reached  various  sen- 
sory cortical  centres.  It  is  evident  that  such  a  combination 
of  impressions  may  also  give  rise  to  various  movements  of 
the  limb,  or  of  the  muscles  of  expression,  and  that  the 
impressions  will  be  more  or  less  vivid  as  the  exciting 
causes  are  strong  or  weak.  If  they  are  vivid,  or,  in  other 
words,  if  the  molecular  changes  in  the  nerve  cells  of  parti- 
cular parts  of  the  cortex  of  the  brain  are  intense,  they  will 
have  both  a  tendency  to  last  after  the  exciting  cause  has 
been  removed,  and  a  tendency  to  be  renewed  by  a  slighter 
stimulus  than  was  at  first  necessary  to  produce  them.  This 
is  the  physiological,  or  organic,  foundation  for  memory,  and 
also  for  the  mental  process  known  as  the  association  of 
ideas.  Further,  if  such  molecular  processes,  by  frequent 
repetition,  stamp  a  certain  character  on  particular  parts  of 
the  cerebral  cortex,  so  as  to  be  transmitted  according  to  the 
laws  of  heredity,  then  we  have  a  physiological  basis  for 
innate  tendencies  or  intuitions.  The  brain  of  one  man 
differs  from  another  in  this  respect.  The  greater  the  num- 
ber and  variety  of  impressions  made  on  an  individual,  the 
greater  will  be  the  number  and  variety  of  the  molecular 
movements  in  the  cells  of  the  cortex,  and  the  greater  the 
number  and  variety  of  resulting  mental  and  reflex  pheno- 
mena. So  intense  may  these  processes  be  that  they  may 
be  called  into  action  by  a  stimulus  from  another  part  of  the 
brain,  as  when  irritation  of  the  corpora  quadrigemina  by 
Indian  hemp  awakens  in  the  cells  of  the  visual  centres  of 
the  cortex  those  changes  which  are  associated  in  the  mind 


296  Physiology  of  the  Senses 

with  long-forgotten  visual  impressions,  and  the  person  sees 
passing  before  him  a  phantasmagoria  of  brilliantly-coloured 
images.  These  may  also  arise  spontaneously,  but  the 
apparent  spontaneity,  however,  is  dependent  on  a  stimulus 
so  feeble  as  to  escape  notice,  as  when  the  sight  of  an  object 
suddenly  and  almost  unconsciously  awakens  memories  of 
past  events,  and  brings  before  the  mind's  eye  forms  and 
colours  that  long  before  produced  impressions  on  the  organs 
of  sense. 

Many  nervous  phenomena  are  at  first  in  a  sense  volun- 
tary, and  by  and  by  they  become  more  and  more  of  a  reflex 
character,  and  are  less  and  less  associated  with  the  higher 
consciousness.  Thus  a  child  acquires  powers  of  walking 
by  repeated  efforts  involving  volition,  judgment,  and  per- 
ception of  different  impressions,  but  the  same  movements 
of  locomotion  may  be  unconsciously  performed  by  an  adult. 
Familiar  examples  also  are  seen  in  the  unconscious  dexterity 
of  movement  of  a  skilful  performer  on  a  musical  instrument, 
or  in  the  deft  movements  of  a  cunning  artificer.  So  is  it 
even  with  psychical  operations  involving  the  action  of  the 
brain,  and  the  brain  cortex  may,  as  in  unconscious  cerebra- 
tion,  pass  through  molecular  processes  which  result  in  the 
unconscious  performance  of  actions  that  would  be  regarded 
as  the  result  of  mental  processes,  if  the  person  were  con- 
scious. Many  instinctive  actions  are  probably  in  this  sense 
of  an  unconscious  character.  There  can  be  no  doubt  that 
even  in  men  the  brain  may  work  unconsciously,  and  the 
product  may  suddenly  start  out  into  consciousness. 

Facility  of  mental  acquirement  means  a  certain  receptive- 
ness  for  particular  kinds  of  molecular  action.  Other  per- 
sonal factors  come  into  operation,  such  as  the  power  of 
choice  of  particular  impressions,  the  degree  of  attentio?i  paid 
to  them  at  the  time  (depending  largely  on  strength  of  will), 
the  degree  of  stability  of  the  results  of  the  molecular  move- 


Physiological  Conditions  of  Sensation        297 

ments  that  have  been  excited,  and  the  power  of  associa- 
tion of  different  impressions.  Each  of  these  factors  has  a 
physiological  basis  peculiar  to  each  individual.  They  are 
susceptible  of  being  extended  and  strengthened  by  exercise, 
and  just  as  muscular  exercise  causes  an  increased  growth 
of  muscular  fibre,  so  regulated  mental  exercise  must  develop 
and  strengthen  the  tissue  of  the  brain.  Thus  one  man 
differs  from  another  in  the  primitive  constitution  of  his 
nerve  centres.  This  determines  his  degree  of  intelligence, 
power  of  accurate  judgment,  and  aptitude  for  special  kinds 
of  work.  These  qualities  are  determined  chiefly  by  inherit- 
a?ice  from  ancestors  who  have  thus  given  their  descendant 
a  groundwork  of  mental  character.  In  the  next  place,  the 
influence  of  a  man's  environment  develops  to  a  greater  or 
less  extent  this  and  that  faculty.  This  is  the  rational  basis 
of  all  educative  processes.  Again,  the  degree  of  excitability 
of  the  nerve  centres  varies  considerably  among  individuals, 
and  it  also  may  be  influenced  by  exercise.  On  this  depends 
the  aptitude  for  reflex  acts  of  all  kinds.  Lastly,  there  may 
be  a  greater  or  less  influence  exerted  by  the  higher  over  the 
lower  centres,  or,  in  other  words,  a  greater  or  less  degree 
of  inhibitory  power.  The  power  of  the  will,  which  may 
also  be  strengthened  by  exercise,  or  weakened  by  yielding 
to  disease,  or  by  tame  compliance,  depends  on  this  factor. 
Thus  by  a  study  of  nervous  actions,  as  connected  with  and 
stimulated  by  impressions  on  the  organs  of  sense,  we  have 
constructed  a.  physiological  basis  of  character,  and  that  with- 
out admitting  the  truth  of  an  exclusively  materialistic  hypo- 
thesis. Behind  all  brain  action,  although  closely  connected 
with  it,  there  is  the  strongest  probability  of  the  existence  of 
an  immaterial  agent  of  which  Spenser  wrote  in  his  Hy?H?i 
in  Honour  of  Beauty  : 

"  For  of  the  soul  the  body  form  doth  take, 
For  soul  is  form,  and  doth  the  body  make." 


APPENDIX    I 

THE  ACTION  OF  LIGHT  ON  THE  RETINA 

At  p.  150  reference  is  made  to  the  electrical  change  that  occurs 
when  light  falls  on  the  living  retina.  A  full  description  of  this 
remarkable  phenomenon  was  out  of  place  at  that  part  of  the  book, 
but  inasmuch  as  it  is  the  only  example  we  have  of  a  known  physical 
process  occurring  in  a  terminal  organ  of  sense,  it  merits  here  a 
further  notice.  For  the  detection  of  electrical  currents  in  living 
tissues  a  sensitive  galvanometer  of  high  resistance  must  be  employed. 
The  currents  are  led  off  the  living  tissues  by  electrodes  that  are  so 
constructed  as  to  be  unpolarisable — that  is  to  say,  they  do  not  them- 
selves generate  any  current,  nor  are  they  altered  by  the  passage  of 
even  a  feeble  current  through  them,  so  as  to  give  rise  to  any  electrical 
action.  They  simply  lead  off  to  the  galvanometer  any  current  that 
may  exist.  Such  electrodes  are  variously  constructed  ;  but  a  con- 
venient form  is  a  trough  of  zinc,  resting  on  insulating  plates  of 
vulcanite,  amalgamated  on  the  inner  surface,  and  filled  with  a 
saturated  solution  of  sulphate  of  zinc.  A  pad  of  blotting-paper,  wet 
with  the  sulphate  of  zinc  solution,  is  placed  into  each  trough,  and 
on  the  pad  a  bit  of  clay,  moistened  with  saliva,  is  laid,  so  as  to  pro- 
tect any  animal  tissues  placed  on  the  clay  from  the  irritant  action  of 
the  sulphate  of  zinc.  The  electrodes,  so  prepared,  are  connected 
with  the  galvanometer.  A  frog's  eye  is  dissected  out  (after  the 
animal  has  been  decapitated,  and  all  sensation  has  been  lost),  and  is 
so  placed  on  the  pads  of  clay  that  one  pad  touches  the  middle  of  the 
surface  of  the  cornea,  and  the  other  the  posterior  surface  of  the  eye- 
ball and  the  transverse  section  of  the  optic  nerve.  A  current,  which 
we  may  call  the  "resting-eye  current,"  is  shown  by  a  deflection  of 
the  needle  of  the  galvanometer.     It  can  be  shown  that  this  current 


300  Physiology  of  the  Senses 

passes  from  the  corneal  surface  through  the  galvanometer  and  back 
to  the  posterior  surface  of  the  eyeball — that  is  to  say,  the  eyeball 
acts  like  a  little  galvanic  element,  the  positive  pole  of  which  is  the 
cornea  and  the  negative  pole  the  transverse  section  of  the  optic 
nerve.  The  eye  is  now  covered  with  a  blackened  box  so  as  to  keep 
it  in  the  dark,  and  the  box  is  provided  with  a  shutter  by  which  the 
light  may  be  shut  off  or  admitted  at  pleasure.  When  we  open  the 
shutter,  and  allow  light  to  fall  upon  the  eye,  the  needle  of  the  gal- 
vanometer will  be  seen  to  swing  in  the  direction  that  indicates  an 
increase  in  the  current.  If  light  is  allowed  to  act  on  the  eye  for 
a  few  minutes,  the  current  diminishes,  falls  off  in  strength  as  the 
retina  becomes  fatigued,  and  soon  becomes  less  than  it  was  when 
light  was  allowed  to  fall  on  the  eye.  If  the  light  is  allowed  to  act 
sufficiently  long,  the  current  becomes  less  and  less  until  it  reaches 
zero.  If,  however,  we  remove  the  light  by  closing  the  shutter  before 
the  retina  has  become  too  fatigued,  there  is  at  once  a  second  in- 
crease in  the  strength  of  the  current  again  indicated  by  a  swing  of 
the  galvanometer  needle,  then  a  rapid  diminution,  and  soon  the 
needle  becomes  almost  stationary.  These  are  the  details  of  a  single 
experiment ;  and  they  show  that  light  alters  the  electrical  condition 
of  the  eye,  the  impact  of  light  causing  an  increase,  its  continued 
action  a  diminution,  and  its  removal  another  increase  in  the  ' '  resting  - 
eye  current." 

It  can  be  shown  that  the  effect  is  due  to  the  action  of  light  on  the 
retina,  because  if  this  structure  be  removed,  light  will  produce  no 
variation  in  any  current  that  may  be  got  from  other  structures.  The 
effect  is  due  to  light  and  not  to  heat,  because  it  is  easy  to  absorb  the 
heat  rays,  and  still  allow  the  light  to  pass,  and  vice  versd.  In  both 
cases  it  is  only  when  light  rays  reach  the  retina  that  the  effect  is 
obtained.  These  variations  have  been  seen  in  the  eyes  of  inverte- 
brates and  vertebrates,  and  even  in  the  eye  of  man  himself.  Further, 
by  allowing  the  different  rays  of  the  spectrum  to  fall  on  the  eye,  we 
can  show  that  the  luminous  yellow  rays  produce  more  effect  than 
the  less  luminous  green,  red,  blue,  or  violet  rays,  and  that  the  sum 
of  the  effects  of  the  different  rays  is  almost  that  of  white  light.  It  can 
also  be  demonstrated  that  the  effects  of  varying  intensities  of  light 
agree  with  the  laws  formulating  the  relation  between  the  strength  of 
the  stimulus  and  the  strength  of  the  resulting  sensation  referred  to 
on  P*  39-  The  importance  of  this  observation  is  due  to  the  indica- 
tion it  gives  that  the  stimulus-sensation-ratio  may  be  a  function  of 
the  terminal  organ  as  well  as  of  the  brain. 


Appendix  I  301 

The  electrical  variations  above  described  may  be  physical 
indications  of  chemical  phenomena  known  to  occur  in  the  retina. 
This,  however,  has  not  been  proved.  It  is  conceivable,  as  an  alter- 
native hypothesis,  that  the  rods  and  cones  act  as  transforming  struc- 
tures, changing  the  waves  of  light  into  electrical  variations  that  pass 
along  the  fibres  of  the  optic  nerve.  Electrical  variations  are  the 
only  phenomena  that  have  yet  been  demonstrated  in  a  nerve  fibre 
during  the  passage  along  it  of  a  nervous  impulse ;  and  if,  as  the 
physicists  assert,  light  waves  are  only  short  electrical  waves,  the 
hypothesis  suggested  is  not  unreasonable. 

These  electrical  changes  in  the  retina,  caused  by  the  action  of 
light,  were  independently  discovered  by  Holmgren  in  Upsala,  and 
by  Dewar  and  M'Kendrick  in  Edinburgh,  between  1870  and  1873.1 

1  Dewar  and  M'Kendrick,  Proceedings  of  Royal  Society  of  Edin- 
burgh, 1874.  Also  M'Kendrick's  Text -Book  of  Physiology,  vol.  ii. 
p.  627. 


APPENDIX    II 

DERIVATIONS  OF  SCIENTIFIC  TERMS 

ABERRATION,  L.  ab,  away ;  erro,  erratum,  to  wander 

Actinic,  Gr.  aktis,  a  sunbeam 

Acustica,  Gr.  akouo,  to  hear 

■ffisthesiometer,  Gr.  cesthesis,  feeling ;  metron,  a  measure 

Afferent,  L.  ad,  to  ;  fero,  I  carry 

Alkaloid,  Arab,  alkali ;  Gr.  eidos,  likeness 

Allotropic,  Gr.  allotropos,  of  a  different  nature 

Ametropia,  Gr.  a,  not ;  metron,  measure ;  ops,  the  eye 

Amplitude,  L.  amplitudo,  largeness 

Ampulla,  L.  ampulla,  a  bottle 

Ansesthesia,  Gr.  a,  without ;  astkesis,  perception 

Analgesia,  Gr.  a,  without ;  algos,  pain 

Anode,  Gr.  ana,  up  ;  hodos,  a  way 

Anosmia,  Gr.  a,  without ;  osme,  smell 

Aqueous,  L.  aqua,  water 

Arborescent,  L.  arboresco,  to  become  a  tree 

Astigmatism,  Gr.  a,  without ;  stigma,  a  point 

Ataxia,  Gr.  a,  without ;  taxis,  arrangement 

Auditory,  L.  audio,  auditum,  to  hear 

Aura,  Gr.  ao,  to  breathe 

Auricle,  L.  auriculus,  dim.  of  auris,  an  ear 

Automatic,  Gr.  automatos,  of  one's  own  accord 

BASSOON,  Gr.  basis,  base  ;  a  wind  instrument  giving  a  low  note 
Biconvex,  L.  bis,  twice  ;  con,  together ;  veho,  vectum,  to  carry 
Binary,  L.  bina,  a  pair 


Appendix  II  3°3 

Binaural,  L.  bis,  twice  ;  audio,  I  hear 
Binocular,  L.  bis,  twice ;  oculus,  the  eye 

CALLOSUM,  L.  callosus,  thick-skinned 

Camera,  L.  camera,  a  chamber 

Capillary,  L.  capillus,  a  hair 

Cardinal,  L.  cardo,  a  hinge 

Cataract,  Gr.  kata,  down ;  arasso,  to  fall 

Cerebellum,  L.  cerebelhim,  dim.  of  cerebrum,  the  little  brain 

Cerebrum,  L.  cerebrum,  the  brain 

Cerumen,  L.  ££nz,  wax 

Choroid,  Gr.  ckorioti,  skin  ;  £z<afo.y,  likeness 

Chromatic,  Gr.  chroma,  colour 

Ciliary,  L.  cilium,  an  eyelash 

Cilium  {pi.  cilia),  L.  ciluim,  an  eyelash 

Circumvallate,  L.  circum,  around ;  vallum,  a  wall 

Cochlea,  Gr.  kochlias,  a  snail  with  a  shell 

Coma,  Gr.  koma,  drowsiness 

Commissure,  L.  com,  together ;  mitto,  missum,  to  send 

Complementary,  L.  com,  together ;  pleo,  to  fill 

Congenital,  L.  congenittis,  born  together  with 

Conjugate,  L.  con,  together  ;  jugum,  a  yoke 

Conjunctiva,  L.  con,  together  ;  jungo,  juncttim,  to  join , 

Consciousness,  L.  con,  together ;  scio,  I  know 

Convergence,  L.  con,  together ;  vergo,  to  bend 

Convolution,  L.  convolvo,  convolutum,  to  roll 

Corium,  Gr.  chorion,  skin 

Cornea,  L.  cornu,  a  horn 

Corona,  L.  corona,  a  crown 

Corpus  {pi.  corpora),  L.  corpus,  a  body 

Corpuscle,  L.  corpuscuhis,  dim.  of  corpus,  a  body 

Cortex,  L.  cortex,  bark 

Cranium,  Gr.  kranion,  the  skull 

Cribriform,  L.  cribrum,  a  sieve  ;  forma,  likeness 

Crista,  L.  crista,  a  crest 

Cuneus,  L.  cuneus,  a  wedge 

Cupula,  L.  cupula,  a  small  cup 

DALTONISM,  Dalton,  a  celebrated  chemist  who  was  colour-blind 
Decussation,  L.  decusso,  to  place  crosswise  in  the  form  of  an  X 
Dental,  L.  dens,  dentis,  a  tooth 


304  Physiology  of  the  Senses 

Derma,  Gr.  derma,  the  skin 
Diabetes,  Gr.  dia,  through ;  baino,  to  go 
Diaphragm,  Gr.  dia,  across  ;  phrasso,  to  fence 
Dioptrics,  Gr.  di,  through ;  horao,  I  see 
Dispersion,  L.  dis,  asunder ;  spargo,  to  scatter 
Dissonance,  L.  dis,  asunder ;  sonans,  sounding 
Dynamical,  Gr.  dynamis,  power 

EFFERENT,  L.  ex,  out ;  fero,  I  carry 

Electrode,  Gr.  elektron,  amber ;  hodos,  a  way 

Electrolysis,  Gr.  elektron,  amber ;  lysis,  a  softening 

Emmetropic,  Gr.  en,  in ;  metron,  measure ;  ops,  the  eye 

Endolymph,  Gr.  endon,  within ;  lympha,  water 

Entoptic,  Gr.  entos,  within  ;  ops,  the  eye 

Epidermis,  Gr.  epi,  upon ;  derma,  skin 

Epiglottis,  Gr.  epi,  upon ;  glotta,  a  tongue 

Erectile,  L.  e,  out ;  recto,  to  make  straight 

Ether,  Gr.  aither,  the  upper  air 

Ethmoid,  Gr.  etkmos,  a  sieve  ;  eidos,  likeness 

FAUCES,  L.  fauces,  the  gullet 

Fenestra,  L.  fenestra,  a  window 

Fibril,  L.  fibra,  a  filament 

Filament,  L.  filuni,  a  thread 

Filiform,  L.  fihim,  a  thread  ;  forma,  form 

Fluorescence,  L.fuo,  I  flow 

Focus,  L.  focus,  a  fireplace 

Foliata,  L.  folium,  a  leaf 

Follicle,  L.  folliculus,  dim.  oifollis,  a  wind  ball  or  bag 

Foramina,  L.  foro,  to  bore 

Formication,  L.  formica,  an  ant 

Fornicatus,  L.  fornicatus,  arched 

Fovea,  L.  fovea,  a  small  pit 

Function,  L.  fungor,  functum,  to  discharge  an  office 

Fundus,  L.  fundtis,  the  bottom 

Fungiform,  L.  fungus,  a  mushroom  ;  forma,  form 

Fuscin,  "L.ftiscus,  tawny 

GALVANOMETER,  Galvani,  the  discoverer  of  certain  electrical  pheno- 
mena ;  metron,  a  measure 
Ganglion  {pi.  ganglia),  Gr.  ganglion,  a  tumour  under  the  skin 
Glossopharyngeal,  Gr.  glossa,  the  tongue  ;  pharynx,  the  throat 


Appendix  II  305 

Gustatory,  L.  gztstatus,  tasted 
Gyri,  Gr.  gyros,  a  circuit 

HEMORRHAGE,  Gr.  hairnet,,  blood  ;  rheo,  to  flow 

Hamulus,  L.  dim.  of  hamus,  a  hook 

Helicotrema,  Gr.  helix,  a  spiral ;  trema,  a  perforation 

Hemianesthesia,  Gr.  hemi,  half;  a,  without;  eesthesis,  feeling 

Heteronomous,  Gr.  heteros,  another  ;  onoma,  a  name 

Hippocampus,  Gr.  hippos,  a  horse  ;  kampos,  a  sea-monster 

Homologous,  Gr.  homos,  the  same ;  logos,  a  discourse 

Homonomous,  Gr.  homos,  the  same ;  oitoma,  a  name 

Horopter,  Gr.  horos,  a  boundary ;  opter,  a  spectator 

Hyaloid,  Gr.  hyalos,  glass ;  eidos,  a  likeness 

Hypermetropia,  Gr.   hyper,  beyond ;   metron,   measure ;  ops,   the 

eye 
Hypnotism,  Gr.  hypnos,  sleep 
Hypoglossal,  Gr.  hypo,  under ;  glossa,  the  tongue 
Hypometropia,  Gr.  hypo,  under  ;  metron,  measure ;  ops,  the  eye 

ILLUSION,  L.  in,  in  ;  ludo,  lusum,  to  play 

Incus,  L.  mats,  an  anvil 

Index,  L.  in,  in ;  dico,  to  proclaim 

Internuncial,  L.  inter,  between ;  mtntius,  a  messenger 

Intuition,  L.  intus,  within ;  itum,  to  go 

Iris,  Gr.  iris,  the  rainbow 

JAUNDICE,  Fr.  jaune,  yellow 

KATHODE,  Gr.  kata,  down  ;  hodos,  a  way 
Klang,  Ger.  klang,  the  quality  of  a  sound 

LABYRINTH,  Gr.  labyrinthos,  a  labyrinth 
Lachrymal,  L.  lachryma,  a  tear 
Lamella,  L.  lamella,  dim.  of  lamina,  a  small  plate 
Lamina,  L.  lamina,  a  small  plate 
Laxator,  L.  laxo,  to  loosen 
Lens,  L.  lens,  a  lentil 
Lenticular,  L.  dim.  of  lens,  a  small  bean 
Limbus,  L.  limbtts,  a  border 
Lingual,  L.  lingua,  a  tongue 
Logarithm,  Gr.  logos,  ratio ;  arithmos,  number 

X 


306  Physiology  of  the  Senses 

Lumen,  L.  lumen,  light 
Luminosity,  L.  lumen,  light 

MACERATE,  L.  macero,  to  waste  away 

Macula,  L.  macula,  a  spot 

Malleus,  L.  malleus,  a  hammer 

Mastoid,  Gr.  mastos,  the  breast 

Meatus,  L.  meo,  meaitim,  to  pass 

Medulla,  L.  medulla,  the  marrow ;  medius,  the  middle 

Melanin,  Gr.  melan,  black 

Meridional,  L.  meridies,  midday 

Mesentery,  Gr.  mesos,  middle ;  enteros,  intestines 

Minimum  visibile,   L.  minimum,  the  least ;   visibile,  able  to  be 

seen 
Modiolus,  L.  dim.  of  modus,  a  measure 
Molecular,  L.  dim.  of  moles,  a  mass 
Momentum,  L.  moveo,  to  move 
Morphological,  Gr.  morphe,  form ;  logos,  a  discourse 
Motor,  L.  moveo,  motum,  to  move 
Mucus,  Gr.  muxa,  the  mucus  of  the  nostrils 
Muscae  volitantes,  L.  musca,  a  fly  ;  volitans,  flying 
Myopia,  Gr.  muo,  to  close  ;  ops,  the  eye 

NARES,  L.  nares,  the  nostrils 

Neurilemma,  Gr.  neuron,  a  nerve  ;  lemma,  a  coat 

Neuro- epithelium,   Gr.    neuron,  a  nerve  ;   epi,  upon  ;   tithemi,  to 

place 
Neuroglia,  Gr.  neuron,  a  nerve ;  glia,  glue 
Nexus,  L.  necto,  to  twine 
Nodal,  L.  nodus,  a  knot 
Nucleus,  L.  nucleus,  the  kernel 

OCCIPITAL,  L.  tf£,  against ;  ra/«*,  the  head 

Odoroscope,  L.  <?afcr,  odour  ;  Gr.  skopeo,  I  sec 

Olfactory,  L.  olfacio,  to  smell 

Operti,  L.  opertus,  opened 

Ophthalmic,  Gr.  ophthahnos,  the  eye 

Ophthalmoscope,  Gr.  ophthalmos,  the  eye ;  skopeo,  I  see 

Orbit,  L.  orbita,  an  orbit 

Organ,  Gr.  organon,  an  instrument 

Organism,  Gr.  organon,  an  instrument 


Appendix  II  307 

Ossicle,  L.  dim.  of  os,  a  bone 
Otoconia,  Gr.  ous,  otos,  the  ear  ;  konis,  dust 
Otolith,  Gr.  ous,  otos,  the  ear ;  lithos,  a  stone 
Ozone,  Gr.  ozo,  to  smell 

Pancreas,  Gr.  pan,  all ;  kreas,  flesh 
Papilla,  L.  papilla,  a  nipple 
Parietal,  L.  paries,  a  wall 
Pari  passu,  L.  par,  equal ;  passus,  step 
Pathological,  Gr.  pathos,  suffering  ;  logos,  a  discourse 
Peduncle,  L.  pedo,  having  broad  feet 
Pellicle,  L.  pellicula,  dim.  oipellis,  a  skin 
Period,  Gr.  periodos,  a  going  round 
Peripheral,  Gr.  periphereia,  a  periphery 
Peritoneum,  Gr.  peritonaios,  stretched  over 
Petrous,  Gr.  petra,  a  rock 

Phakoscope,  Gr.  phakos,  a  lentil,  the  lens  ;  skopeo,  I  see 
Phalangse,  Gr.  phalanx,  a  block 

Phantasmagoria,  Gr.  phantazo,  to  make  appear ;  agora,  an  assembly 
Pharynx,  Gr.  pharynx,  the  throat 
Phase,  Gr.  phasis,  phaino,  to  show- 
Phenomenon,  Gr.  phainomenon,  appearing 
Photometrical,  Gr.  phos,  light ;  metron,  a  measure 
Physharmonica,  Gr.  physao,  to  blow  ;  harmonikos,  musical 

Pigment,  L.  pingo,  to  paint 

Pitch,  A.  S.  pycan,  to  pick  or  strike  with  a  pike 

Pituita,  L.  pituita,  phlegm 

Plane,  L.  planus,  smooth 

Plexus,  L.  plexus,  a  network 

Pons,  L.  pons,  a  bridge 

Precuneus,  L.  pr&,  before  ;  ctmeus,  a  wedge 

Presbyopia,  Gr.  presbys,  old  ;  ops,  the  eye 

Prism,  Gr.  prisma,  from  prio,  to  saw 

Protoplasm,  Gr.  protos,  first ;  plasma,  anything  formed 

Pseudoscope,  Gr.  pseudos,  false  ;  skopeo,  I  see 

Psychical,  Gr.  psyche,  the  soul 

Pupil,  L.  pupilla,  dim.  of  pupa,  a  puppet 

QUADRIG-EMINA,  L.  quahwr,  four  ;  gemini,  double 
Quantum,  L.  quatitum,  how  much 


308  Physiology  of  the  Senses 

RECTUS,  L.  rectus,  straight 

Refraction,  L.  re,  back  ;  frango,  fractum,  to  break 

Refrangible,  L.  re,  back ;  frango,  to  break 

Resonator,  L.  re,  again ;  sono,  to  sound 

Reticulated,  L.  rete,  a  net 

Retina,  L.  rete,  a  net 

SACCHARINE,  L.  saccharum,  sugar 

Saccule,  L.  dim.  of  saccus,  a  bag 

Schematic,  Gr.  schema,  form 

Sclerotic,  Gr.  skleros,  hard 

Sebaceous,  L.  sebum,  suet 

Section,  L.  j^rc,  sectum,  to  cut 

Segment,  L.  ^^,  to  cut 

Sensorium,  L.  sentio,  sensum,  to  feel 

Septum,  L.  sepes,  a  hedge 

Serous,  L.  serum,  a  watery  fluid 

Sine,  L.  sinus,  a  curve 

Spectrum,  L.  specio,  I  see 

Sphenoid,  Gr.  sphen,  a  wedge ;  £&a?iw,  likeness 

Sphincter,  Gr.  sphingo,  I  contract 

Squamous,  L.  squama,  the  scale  of  a  fish 

Stapes,  L.  stapes,  a  stirrup 

Stereoscope,  Gr.  stereos,  solid ;  skopeo,  I  see 

Stimulus,  L.  stimulus,  a  goad 

Striata,  L.  striatum,  grooved 

Stylet,  Gr.  stylos,  a  style  or  pencil 

Sulcus,  L.  sulcus,  a  groove 

Synchronous,  Gr.  syn,  together ;  chronos,  time 

Syren,  L.  siren,  a  singer  of  sweet  music 

TAPETUM,  Gr.  tapes,  tapestry 

Telestereoscope,   Gr.   tele,  at  a  distance  ;  stereos,  solid ;  skopeo,   I 

see 
Temporo-sphenoidal,  L.  tempora,  the  temples  ;  Gr.  sphen,  a  wedge  ; 

mfo.r,  likeness 
Thalamus,  Gr.  thalamos,  a  couch 
Thaumatrope,  Gr.  thauma,  wonder  ;  tropos,  a  turning 
Timbre,  Fr.  timbre,  the  sound  of  a  bell,  the  voice 
Translucent,  L.  /ra;z.y,  through  ;  luceo,  to  shine 
Triturate,  L.  tritus,  rubbed 


Appendix  II  309 


Turbinated,  L.  turbinates,  pointed 
Tympanum,  Gr.  tympanon,  a  drum 

UMBO,  L.  umbo,  the  boss  of  a  shield 
Uncinate,  L.  uncus,  a  hook 
Undulatory,  L,  unda,  a  wave 
Utricle,  L.  dim.  of  uter,  a  leathern  bag 
Uvula,  L.  dim.  of  uva,  a  grape 

VAS,  L.  vas,  a  vessel 

Vertebrate,  L.  verto,  I  turn 

Vestibule,  L.  vestibulum,  a  threshold 

Vibration,  L.  vibro,  to  quiver 

Vibrissse,  L.  vibro,  to  quiver 

Vidian,  after  Vidius,  who  described  the  Vidian  nerve 

Viscera,  L.  viscera,  the  bowels 

Vitreous,  L.  vitrum,  glass 

Volatility,  L.  volo,  volatum,  to  fly 

Vorticosa,  L.  verto,  to  turn 

ZERO,  Arab,  tsaphara,  empty 
Zonule,  L.  dim.  of  zona,  a  belt 


INDEX 


Aberration,  spherical,  122  ; 
chromatic,  124  ;  chromatic,  of 
eye,  131  ;  spherical,  of  eye, 
132 

Abney  on  colour  vision,  170 

Absolute  sensitiveness,  56 

Accommodation  of  eye  for  dis- 
tance, 135 

Aerial  perspective,  188 

^Esthesiometer,  55 

After-image,  153  ;  positive,  154  ; 
negative,  154  ;  coloured,  161 

After-tactile  impressions,  58 

Ajugari,  Lucrezia,  voice  of, 
246 

Albinos,  101 

Ametropic  eye,  138 

Ampulla,  224 

Anaesthesia,  16 

Analgesia,  16 

Analogy  between  touch  and  hear- 
ing, 53 

Angle  of  convergence,  188 

Anosmia,  94 

Antennas  of  insects,  52 

Apex-process,  28 

Appendages  of  the  skin,  43 

Aqueous  humour,  100 

Area  of  distinct  vision,  145 

Arensohn  on  odours,  91,  92 

Aristotle's  experiment,  61 

Aromatic  bodies,  87 

Association,  fibres  of,  295  ;  of 
ideas,  295 


Astigmatism,  132 

Auditory  hairs,  227  ;  nerve,  223  ; 

teeth,  235 
Aura  of  epilepsy,  33 
Auricle,  200  ;  its  function,  201 
Automatic  movements,  20 

Balfour,  F.  M. ,  on  sensory 
apparatus,  8 

Beats,  259 

Beat-tones,  260 

Beaunis  on  odours,  92 

Binaural  audition,  283 

Binocular  vision,  170  ;  visual  field, 
177 

Birds,  cochlea  of,  271 

Blindness,  psychical,  31 

Blind  spot,  149 

Bowman,  glands  of,  in  nose,  85  ; 
spiral  ligament  of,  229  ;  ante- 
rior and  posterior  elastic  lamina 
of,  99 

Brightness  of  colour,  159 

Brown,  A.  Crum,  on  semicircular 
canals,  268 

Bruch,  membrane  of,  102 

Bulb,  18 

Calloso-marginal  fissure,  27 

Canalis  reuniens,  225 

Canals,  semicircular,  their  de- 
velopment, 224;  in  equilibrium, 
267 

Cardinal  points,  125 


312 


Physiology  of  the  Senses 


Cataract,  107 

Cells  of  cortex  of  brain,  29 

Centre  for  hearing,  32  ;  for  per- 
ception of  heat  and  cold,  35  ; 
of  rotation  of  eyes,  171  ;  for 
taste  and  smell,  34  ;  for  touch, 
34;  for  vision  in  cortex  cerebri,  30 

Cerebellum,  19 

Cerebral  peduncles,  22 

Cerebration,  unconscious,  296- 

Cerebrum,  22 

Cerumen,  204 

Chain  of  bones,  209  ;  movements 
of,  211 ;  transmission  of  vibra- 
tions by,  218 

Chamber,  anterior,  100  ;  pos- 
terior, 108 

Choice,  power  of,  296 

Chorda  tympani,  205 

Choroid,  99 

Ciliary  arteries  and  veins,  100  ; 
ganglion,  in  ;  muscle,  102  ; 
nerves,  m;  processes,  the, 
102 

Circle  of  sensibility,  62 

Claudius's  cells,  236 

Cleland,  theory  as  to  seat  of 
consciousness,  288 

Cochlea,  228  ;  its  function,  273 

Cochlear  canal,  225,  228,  230 

Cold  spots,  64 

Colour  blindness,  159 

Colour,  sensation  of,  155 

Colour  of  the  skin,  43 

Colour  vision  theories,  161 

Coma,  286 

Common  sensations,  35 

Compasses  for  touch,  54 

Cones  of  retina,  103,  104 

Confusion  colours,  160 

Conjunctiva,  99 

Consciousness,  286  ;  seat  of,  288  ; 
not  a  form  of  energy,  291 

Contrast  of  colours,  161 

Convolutions  of  brain,  24 

Co-ordinated  movements,  294 

Corium,  41 

Cornea,  98 


Corona  radiata,  28 
Corpora  quadrigemina,  22 
Corpus  callosum,   24  ;   striatum, 

23.  35 
Corti,  organ  of,  232 
Cribriform  plate  of  ethmoid  bone, 

83 
Crista  acustica,  227 
Crystalline  lens,  106,  107 
Cuneus,  28 
Cupula,  227 
Cutis  vera,  41 
Cyon  on  semicircular  canals,  267 

Daltonism,  159 
Deafness  resulting  from  destruc- 
tion of  cortical  centre,  34 
Decussation  of  nerve  fibres,  15 
Degeneration  of  nerve  fibre,  13 
Deiter's  cells,  235  ;  their  func- 
tion, 266 
Delicacy  of  sense  of  smell,  92 
Derma,  41,  42 

Descemet,  membrane  of,  99 
Dewar,   observations   on   physio- 
logical action  of  light,  301 
Dioptrics,  laws  of,  115 
Dissonance,  260 
Distance,  estimation  of,  187 
Distinct  vision,  175 
Donders  on  the  eye,  171 
Drum,  drum-head,  199,  202 
Ductus  endolymphaticus,  225 

Ear,  external,  200  ;  middle,  204 ; 
internal,  223  ;  their  functions, 
264,  265 

Emmetropic  eye,  138 

End-bulbs,  47 

Endolymph,  266 

Entoptic  phenomena,  141 

Epidermic  structures,  their  func- 
tions, 43,  51 

Epidermis,  41,  42 

Epithelium,  olfactory,  84 

Ethmoid  bone,  81 

Eulenberg,  sensitiveness  of  skin, 
57 


Index 


3i3 


Eustachian  tube,  82,  199,  207 
Externality  in  sensation,  40 
Eye,  adjustment  for  different  dis- 
tances,   134 ;    examination   of 
interior  of,    143  ;   dioptric  sys- 
tem of,  127  ;  its  defects  as  an 
optical  instrument,  131 
Eyeball,    structure  of,    97  ;    con- 
tents of,  105 

Fatigue  of  nerve,  4 
Fechner's  law  of  sensation,  39 
Fenestra   ovalis,    206  ;    rotunda, 

207,  229 
Fenestrated  membrane,  236 
Ferrier  on  brain,  29,  34 
Festing  on  colour  vision,  170 
Fissure    of    Rolando,     26 ;     of 

Sylvius,  26 
Flavour,  74 

Flowers,  and  odours,  89 
Fluorescence,  116 
Focal  points,  125,  128 
Focus,   principal  and  conjugate, 

120 
Follicle  of  hair,  50 
Foramina  nervina,  237 
Form,  judgment  as  to,  194 
Formication,  35 

Forster,  Gaspard,  voice  of,  245 
Fovea  centralis,  105 
Fritsch  on  brain,  29 
Frontal  lobe  of  brain,  26 
Fundamental  colours,  158  ;  tone, 

250,  251,  253 
Fuscin,  105 
Fusion  of  tactile  impressions,  58 

Galton's   observations    on    the 

blind,  56 
Ganglia,  9  ;  spiral  ganglion,  237 
Gaspard  Forster,  basso,  245 
Gauss,  cardinal  points  of,  125 
Glaser,  fissure  of,  205 
Glosso-pharyngeal  nerve,  72,  73 
Goldscheider,    hot    and    cold 

spots,  64 
Graham  on  odours,  93 


Grandry's  corpuscles,  46 

Gratiolet,  radiation  of,  30,  32 

Gustatory  nerves,  72 

Gymnema  sylvestre,  j'j 

Gyri  operti,  26 

Gyrus,  a  convolution  of  brain,  24 ; 
fornicatus,  27,  35  ;  hippo- 
campus, 27  ;  a  centre  for 
touch,  35  ;  uncinatus,  a  centre 
for  smell,  34 

Hair-cells,  inner,  233  ;  outer, 
235  ;  their  function,  266 

Hall,  Stanley,  theory  of  colour 
vision,  164 

Hallucinations,  auditory,  279 

Hamulus,  228 

Harmonic  motion,  248 

Harmonics,  253 

Hartmann,  Von,  287 

Hearing,  198,  centre  in  cerebrum 
for,  32  ;  range  of,  245 

Hearing  affected  by  drugs,  278 

Helicotrema,  228 

Helmholtz,  Von,  theory  of  colour 
vision,  162,  169  ;  ophthalmo- 
scope of,  143  ;  telestereoscope 
of,  184 ;  resonators  of,  251  ; 
syren  of,  242  ;  on  quality  of 
sounds,  256,  257,  260  ;  theory 
as  to  function  of  cochlea,  271 

Hemianesthesia,  35 

Henry,  Ch.  ,  on  odours,  86,  94 

Hensen's  spot,  235  ;  cells,  236  ; 
on  Mysis,  265 

Herbst's  corpuscles,  50 

Hering,  theory  of  colour  vision, 

165 

Heteronomous  images,  179 

Hitzig  on  brain,  29 

Holmgren,  observations  on  phy- 
siological action  of  light,  301 

Homonomous  images,  178 

Horopter,  178 

Horsley,  areas  of  brain,  29,  34,35 

Hot  spots,  64 

Hue  of  colour,  159 

Hyaloid  membrane,  105 


3i4 


Physiology  of  the  Senses 


Hypermetropic  eye,  138 
Hypometropic  eye,  138 

Illusions  of  vision,  192 

Images  formed  by  lenses,  120, 
121 

Incus,  210 

Intensity,  246  ;  of  odours,  92  ;  of 
sensation,  37,  38  ;  of  taste, 
76 

Internal  capsule,  23 

Intuitions,  295 

Iris,  the,  100 

Iridescence  of  epidermic  struc- 
tures, 45 

Irradiation,  140,  154 

Island  of  Reil,  26 

Jacob's  membrane,  103 

Klang  of  musical  tone,  247 
Koenig,   analysis  of  compound 

tones,  254 
Krause's  end-bulbs,  47  ;  theory 

as  to  touch,  63 

Labyrinth,   membranous,    199, 

225  ;  osseous,  223 
Lachrymal  gland,  97 
Ladd  on  colour  sense,  168 
Lambert  on  colours,  156 
Lamina  cribrosa  of  the  eye,  109  ; 

spiralis     ossea,     228  ;      mem- 

branacea,  230 
Langerhans,  cells  of,  45 
Laxator  tympani,  205 
Le  Conte,   divergence  of  visual 

axes,  174 
Lens,  biconvex,  119 
Lenticular  process  of  incus,  210 
Liegeois  on  odours,  90 
Light,    physiological    action    of, 

299;  physical  nature  of,   115; 

reflection  and  refraction  of,  116 
Limbus,  230 
Line    of     regard,    131  ;     vision, 

131 

Listing,  cardinal  points  of,  125  ; 


schematic  and  reduced  eye  of, 

130 
Lobes  of  the  brain,  26 
Locomotor  ataxia,  17 
Loudness  of  sound,  246 
Lower  limit  of  excitation,  37 
Lucrezia     Ajugari,     soprano, 

246 
Luminiferous  ether,  97,  115 

Mach,    action    of     semicircular 

canals,  268 
Macula  acustica,  226 
Majendie,     paths     of    sensory 

fibres,  10 
Malleus,  209 

Malpighi,  stratum  of,  42 
Marginal  gyrus,  27 
Massiveness  of  taste,  75 
Manubrium  of  malleus,  209 
M'Kendrick,     observations    on 

physiological  action  of  light,  301 
Meatus,  external  auditory,   202  ; 

internal  auditory,  225 
Meatuses  of  nose,  81 
Medulla  oblongata,  18,  19 
Meissner's  touch  corpuscles,  47 
Melanin,  105 
Membrana   basilaris,    230  ;    tec- 

toria,  237  ;  tympani,  199,  202, 

205  ;  response  to  sound-waves, 

214 
Memory,  294  ;  of  sounds,  282 
Merkel's  corpuscles,  46 
Minimum  visibile,  148 
Modiolus,  225,  228 
Mcebius  on  cardinal  points,  125 
Molar  movement,  219,  221 
Molecular  movements,  219 
Motion,  perception  of,  193 
Mueller,  H. ,  sensitive  layer  of 

retina,  143 
Mueller's  sustentacular  fibres, 

103 
Munk,  sensory  centres,  31,  34 
Muscle  volitantes,  141 
Muscles  of  the  eye,  172 
Muscular  sense,  36,  68 


Index 


3i5 


Musical  tones,  240 

Myopic  eye,  138 

Mysis,  experiment  on,  265,  272 

Nares,    anterior   and    posterior, 

81 
Nasal    cartilages,     81  ;     mucous 

membrane,  81  ;  cavities,  80 
Near  point  of  vision,  137 
Nerves,  afferent  and  efferent,  10  ; 

their  structure,  1 1 
Nerve   current,    5  ;    rate   of,    6 ; 

cells,   their  origin,   9  ;   matter, 

3 
Nerves,    fatigue  of  nerve,  4  ;    of 

the  nose,   82  ;    of  the  tongue, 

78 
Nerve-endings  in  the  tactile  hairs, 

50  ;    free,   45  ;    in   corpuscles, 

45 
Neuro-epithelium,  8 

Neuroglia,  28 

Newton,     analysis      of     light, 

116 
NiLSSON,  voice  of,  245 
Nodal  points,  126,  129 
Noises,  240,  262 
Normal  eye,  average,  128 
Nose,  vestibular  portion  of,  83  ; 

respiratory,  83  ;  olfactory,  84  ; 
Nose-leaves  of  bats,  52 

Oblique  muscles,  172 

Occipital  lobe  of  brain,  26 

Occipito- angular  area,  a  visual 
centre,  31  ;  blindness  resulting 
from  destruction  of,  32 

Odoroscope,  90 

Odorous  substances,  their  chemi- 
cal nature,  87 

Odours,  their  influence  on  respira- 
tion, 94  ;  and  heat  absorption, 
89  ;  pure  and  mixed,  93  ;  and 
surface  tension,  90 

Olfactory  cells,  85  ;  epithelium, 
84  ;  lobes,  83  ;  nerves,  83 

Ophthalmoscope,  its  principle, 
143 


Optic  commissure,  109 ;  lobes, 
23  ;  nerve,  109  ;  papilla,  149  ; 
pore,  102  ;  thalami,  23  ;  tracts, 
22,  no 

Orbits,  170 

Organ  of  Corti,  231 

Otoconia,  227 

Otoliths,  227 

Overtones,  253 

Owen  on  tactile  hairs,  52 

Ozone  and  odours,  90 

Pacini's  corpuscles,  46,  48,  49  ; 

their  function,  53 
Pain,  67  ;  its  quality,  68 
Papillae,  42  ;  filiform,  fungiform, 

circum vallate,  70  ;  foliatse,  71 
Parietal  lobe  of  brain,  26 
Partial  tones,  253 
Peduncles,  cerebral,  22 
Perception  time,  7 
Perilymph,  228 
Perspective,  aerial,  188 
Petit,  canal  of,  106 
Phakoscope,  136 
Phalangae,  236 
Phase     of     vibration,     affecting 

quality  of  tone,  257 
Phosgenes,  152 
Pigments,  157 
Pitch  of  musical  tones,  242 
Pituita,  82 
Points,  remote  and  near,  of  vision, 

137 
Pons  Varolii,  21 
Position,     primary,      secondary, 

tertiary,  of  eyeball,  171 
Presbyopic  eye,  140 
Prevost  on  odours,  90 
Prickle  cells  in  skin,  43 
Principal    points,    129  ;     planes, 

125 
Prisms,  if 8 
Processus    cochleariformis,    207  ; 

gracilis  of  malleus,  210 
Promontory,  207 
Protecting    cells    of    taste    bud, 

72 


3i6 


Physiology  of  the  Senses 


Protoplasm,  its   chemical  consti- 
tution, 3  ;  its  instability,  4 
Pseudoscope,  184 
Psychical  blindness,  31 ;  deafness, 

34 
Psycho-physical  time,  6 
Pupil,  100  ;  movements  of,  111 
Purity  of  colour,  159 
Purkinje's  figures,  142 
Purple  of  retina,  151 

Quality  of  musical  tones,  247, 
256  ;  of  sensation,  36 

Quantitative  character  of  sensa- 
tion, 37 

Ramsay  on  odours,  87,  93 

Ray,  course  of,  in  dioptric  system, 
126 

Rectus  muscle,  172 

Reduced  eye,  130 

Reflection  of  rays  of  light,  116 

Reflex  mechanism,  289 

Refraction  of  rays  of  light,  117  ; 
index  of,  119 

Registers  of  voice,  245 

Reil,  island  of,  26 

Reissner's  membrane,  230 

Resolving  power  of  the  eye, 
147 

Resonance,  sympathetic,  255 

Resonators,  251  ;  analysis  of 
tones  by,  252 

Retina,  103  ;  appreciation  of 
colour,  151  ;  fundus  of,  103  ; 
retinal  impressions,  154;  action 
of  light  on,  150  ;  correspond- 
ing points  of,  177  ;  electric 
current  of,  299  ;  rods  of,  103, 
104  ;  examination  of,  143 

Rod  cells  of  taste  bud,  72 

Rods  of  Corti,  232 

Rutherford,  theory  as  to  func- 
tion of  cochlea,  270 

Saccule,  225 

Scala  tympani,  229  ;  vestibuli, 
228 


Schaefer    on    sensory   centres, 

34,  35 

Scheiner's  experiment,  137 

Schematic  eye,  130 

Schlemm,  canal  of,  101 

Schneiderian  membrane,  81 

Schultze  on  odours,  93 

Schwann,  white  substance  of, 
1 1  ;  primitive  sheath  of,  1 1 

Sclerotic,  98 

Semicircular  canals,  their  forma- 
tion, 224  ;  their  function, 
267 

Sensorium,  1 

Sense  of  equilibrium,  270 ;  of 
hearing,  198  ;  of  innervation, 
69  ;  of  locality,  56  ;  of  sight, 
96  ;  of  smell,  80 ;  of  smell, 
its  delicacy,  92  ;  of  taste,  70  ; 
of  temperature,  64  ;  of  touch, 

4i 

Sensibility  of  the  tongue,  78 

Sensitiveness,  absolute,  56  ;  of 
the  skin,  54 

Sensory  paths  in  spinal  cord, 
13  ;  impressions,  objectivity  of, 
40  ;  time  in,  6 ;  mechanism 
of,  1 

Shore  on  tastes,  76 

Sieveking,  55 

Size  of  the  retinal  image,  148  ; 
estimation,  190 

Smell,  cerebral  centres  for,  34  ; 
physical  cause  of,  86  ;  physio- 
logy of,  91  ;  sense  of,  80 

Skin  as  excretory  organ,  43  ; 
structure  of,  41  ;  true  skin, 
42  ;  sensitiveness  of,  54 

Solidity,  perception  of,  180 

Somnambulism,  196 

Sound,  198  ;  its  externality, 
277 ;  its  direction,  280  ;  its 
distance,  281  ;  its  velocity, 
220 

Specific  light  of  the  retina,  152 

Spectrum,  solar,  116 

Spenser,  relation  of  soul  and 
body,  297 


Index 


3*7 


Spinal  cord,  13 

Spiral  ganglion,  237  ;  ligament, 
229,  236 

Stapes,  211 

Stapedius  muscle,  207 

Stereoscope,  181 

Stimulus  and  sensation,  36 

Stirrup-bone,  211 

Stratum  corneum,  of  the  skin, 
42  ;  lucidum  of  the  skin,  43  ; 
mucosum  of  skin,  42 

Structure  of  the  skin,  41 ;  of  cortex 
cerebri,  29 

Subjective  sensations  of  odour, 
94  ;  tastes,  78 

Suelzer  on  taste,  75 

Sulci  of  the  brain,  24 

Sulcus  spiralis,  230 

Supporting  cells  of  olfactory  epi- 
thelium, 85 

Suspensory  ligament  of  lens,  106 

Syren,  242 


Tactile  cells,  simple,  46  ;  com- 
pound, 47  ;  field,  60  ;  hairs, 
51  ;  impressions,  information 
from,  59  ;  organs,  their  struc- 
ture, 45 

Tait,  simple  harmonic  motion, 
248 

Tapetum,  the,  101 

Tartini  on  overtones,  276 

Taste,  physical  causes  of,  73  ; 
solubility  a  condition  of,  73  ; 
physiological  conditions  of,  73  ; 
classification  of,  73  ;  excitants 
of,  75  ;  differentiation  of,  76  ; 
massiveness   of,   75 ;    intensity 

of,  75 
Taste  buds  or  goblets,  71 
Taste  pore,  72 
Telestereoscope,  184 
Temperature,  sense  of,  64 
Temporo  -  occipital    convolution, 

28 
Temporo-sphenoidallobe  of  brain, 

26 


Tensor  tympani,  207  ;  its  func- 
tion, 265 

Terminal  organs,  2 

Test  colours,  160 

Thalami  optici,  23 

Thaumatrope,  154 

Thomson  (Lord  Kelvin),  simple 
harmonic  motion,  248 

Threshold  of  sensation,  37 

Timbre  of  musical  tone,  247 

Tone,  240 

Tongue,  70 

Touch,  sense  of,  41  ;  corpuscles, 
simple,  46  ;  compound,  47  ; 
mechanism  of,  52  ;  theories  as 
to,  62 

Transmission  of  sound  by  cra- 
nium, 222  ;  laws  of,  281 

Tuning-fork,  248 

Tunnel  of  Corti's  organ,  232 

Turbinated  bones  of  nose,  81 

Tympanic  groove,  205 

Tympanum,  199,  204 

Tyndall  on  odours,  89 

Umbo   of   tympanic   membrane, 

206 
Uncinate  gyrus,  27 
Unconscious  cerebration,  296 
Utricle,  224 

Valsalva,  experiments  of,  208 
Vas  spirale,  230 
Vater's  corpuscles,  48 
Vense  vorticosse,  100 
Venturi  on  odours,  90 
Vestibule,  223 
Vibrations  of  strings,  215 
Vibrissas,  tactile  hairs,  51,  81 
Visual    angle,    145,    190 ;    field, 

176 
Vitreous  humour,  106 
Volkmann,  variation  of  acuteness 

of  vision,  150 

Wagner's  touch  corpuscles,  47 

Wave-length,  220 

Weber   on  sensitiveness  of  the 


3i8 


Physiology  of  the  Senses 


skin,    54,    55  ;    theory    as    to 
touch,  62  ;  on  odours,  91 

Wheel  of  life,  154 

Whewell  on  astigmatism,  133 

Yellow  spot,  102 

Young,  Thomas,  theory  of  colour 


vision,  162  ;  undulatory  theory 
of  light,  115 

Zoellner's  lines,  192 

Zone  of  oval  nuclei,  85  ;  of  round 

nuclei,  85 
Zonule  of  Zinn,  106 


THE    END 


Printed  by  R.  &  R.  Clark,  Edinburgh 


r 


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of  Edinburgh ;  Oxford  Lecturer.  Maps  and 
68  Illustrations.      i2mo,  $1.50   net. 

CONTENTS — story  of  nature — substance  of  nature 

— POWER  OF  NATURE — THE  EARTH  A  SPINNING  BALL — THE 
EARTH  A  PLANET — THE  SOLAR  SYSTEM  AND  UNIVERSE — THE 
ATMOSPHERE  —  ATMOSPHERIC  PHENOMENA —  CLIMATES  — THE 
HYDROSPHERE — BED  OF  THE  OCEANS — CRUST  OF  THE  EARTH — 
ACTION  OF  WATER  ON  LAND — RECORD  OF  THE  ROCKS — 
CONTINENTAL  AREA — LIFE  AND  LIVING  CREATURES — MAN  IN 
NATURE — APPENDICES — INDEX. 

tl  An  excellent  book,  clear,  comprehensive,  and  remarkably  accurate.  The 
standard  errors  that  one  has  come  to  expect  in  one  text-book  after  another  are 
successfully  avoided,  and  this  indicates  high  and  scholarly  attainments  on  the 
part  of  the  author,  and  a  broad  acquaintance  with  modern  sources  of  scientific 
statements.  .  .  .  One  who  reaches  a  good  understanding  of  the  book  may 
regard  himself  as  having  made  a  real  advance  in  his  education  towards  an 
appreciation  of  nature."— Professor  W.  M.  Davis,  of  Harvard. 

"  Evidently  prepared  by  one  who  understood  his  subject." — Professor 
James  D.  Dana,  Yale. 

"Admirably  adapted  for  High  Schools,  as  also  a  reference  book  for 
teachers.     I  can  commend  it  with  pleasure."— Professor  S.  W.  Williston. 


UNIVERSITY    MANUALS 


THE   STUDY   OF  ANIMAL   LIFE 

By  J.  Arthur  Thomson,  M.A.,  F.R.S.E.,  University 
of  Edinburgh.     i2mo,  Illustrated,  $1.50   net. 

An  original  and  comprehensive  account  of  all  animal  life,  save 
man.  Such  topics  as  the  wealth  of  life  on  the  earth,  its  distri- 
bution, the  struggle  for  existence,  the  social  and  domestic  life  of 
animals,  instinct,  structure,  heredity,  influence  of  habit  and  sur- 
roundings, etc.,  are  thoroughly  discussed,  though  in  a  bright  and 
interesting  way,  and  with  the  fact  constantly  in  mind  that  the 
book    is   a  manual   and  not  a  cyclopaedia  or  a  special  treatise. 

"  I  have  read  it  with  great  delight.  It  is  an  admirable  work,  giving  a  true 
view  of  the  existing  state  and  tendencies  of  zoology  ;  and  it  possesses  the  rare 
merit  of  being  an  elementary  work,  written  from  the  standpoint  of  the  most 
advanced  thought,  and  in  a  manner  to  be  understood  by  the  beginning  stu- 
dent."—J.  H.  Comstock,  Professor  of  Entomology  in  CornJiill  University, 
and  in  Leland  Stanford  Junior  University. 

"An  interesting  and  stimulating  book,  especially  so  for  teachers.  The 
style  is  clear  and  attractive,  and  the  illustrations  excellent.  The  views  taken 
as  to  evolution  and  heredity  are  sound  and  broad,"— A.  S.  Packard,  Professor 
of  Zoology,  Brown  University, 


THE   ELEMENTS   OF   ETHICS 

An  Introduction  to  Moral  Philosophy.  By  J.  II.  Muirhead, 
M.A.,  Royal  Holloway  College,  England.  121110,  $1.00  net ; 
introduction  price,  80  cents  net. 

Contents  :  Book  I.  The  Science  of  Ethics  :  Problem  of,  Can  there  be  a 
Science  of,  Scope  of  the  Science — Book  II.  Moral  Judgment :  Object  of, 
Standard  of,  Moral  Law — Book  III.  Theories  of  the  End  :  As  Pleasure,  As 
Self-sacrifice,  Evolutionary  Hedonism — Book  IV.  The  End  as  Good  :  As 
Common  Good,  Forms  of  the  Good — Book  V.  Moral  Progress  :  Standard 
As  Relative,  As  Progressive,  As  Ideal — Bibliography. 

"With  admirable  clearness  defines  the  fields,  analyzes  ethical  phenomena, 
subjects  theories  of  various  schools  to  searching  criticism,  and  builds  up  in 
logical  fashion  his  own  system.  An  idealist,  .  ,  .  can  render  good  reasons 
for  the  faith  that  is  in  him.  Spirit  tolerant,  method  scientific,  style  easy  and 
graceful." — Public  Opinion. 

"  The  is  no  other  introduction  which  can  be  recommended.'" 

—  The  Academy,  London. 

Returnable  examination  copy  to  Instructors,  with  view  to 
class  use,  at  Introduction  price. 


UNIVERSITY    MANUALS 


THE   EARTH'S   HISTORY 

An  Introduction  to  Modern  Geology.  By  R.  D.  Roberts, 
M.A. ,  Camb.,  D.  Sc,  Lond.  With  colored  Maps  and  Illustra- 
tions.     l2mo,  $1.50  net. 

The  purpose  of  this  volume  is  to  furnish  a  sketch  of  the  methods  and 
chief  results  of  geological  inquiry,  such  as  a  student,  or  a  reader  interested 
in  the  subject  for  its  own  sake,  would  desire  to  obtain.  It  is  shown  that 
Geology  is  not  a  mere  description  of  rocks  and  fossils,  but  a  history,  and 
the  purpose  of  the  geologist  is  to  reconstruct  from  ancient  fragmentary 
remains  the  old  conditions  that  characterized  successive  stages  of  develop- 
ment— in  a  word,  to  make  out  the  life  history  of  the  earth.  Some  of  the 
problems  are :  the  nature  of  the  crust  movements  to  which  land-areas  and 
mountain  ranges  are  due  ;  -what  was  the  distribution  of  land  and  water  when 
each  group  of  rocks  was  formed ;  what  the  extent  and  contour  of  the  land 
were,  the  condition  of  its  surface  and  the  forms  of  life  ;  what  the  oceanic 
conditions,  depths,  life  inhabiting  the  water,  nature  and  extent  of  the 
materials  brought  down  by  rivers. 

The  records  of  this  series  of  events  are  to  be  found  in  the  successive 
groups  of  rocks,  and  the  chief  object  of  this  volume  is  to  present  in  broad 
outline  results  of  geological  research  which  throw  light  upon  the  structural 
history  of  the  earth,  and  the  method  by  which  that  history  is  worked 
out. 


THE   FRENCH    REVOLUTION 

By   Charles    E.    Mallet,    Balliol    College,    Oxford.       i2mo, 
$1.00  net. 

Contexts  :  Introductory — I.  Condition  of  France  in  the  Eighteenth  Cen- 
tury— II.  Last  Years  of  the  Ancient  Regime — III.  The  Early  Days  of  the 
Revolution — IV.  Labours  of  the  Constituent  Assembly — V.  Parties  and  Poli- 
ticians under  the  Constituent  Assembly — VI.  The  Rise  of  the  Jacobin 
Party — VII.  Influence  of  the  War  upon  the  Revolution — VIII.  Fall  of  the 
Gironde — IX.  The  Jacobins  in  Power — X.  The  Struggle  of  Parties  and 
the  Ascendency  of  Robespierre — XL  The  Reaction — Tables  of  Dates — 
Appendix  of  Books — Index. 

This  book  has  a  special  value  to  students  and  readers  who  do  not  own  the 
great  works  of  such  writers  as  De  Tocqueville,  Taine,  Michelet,  and  Von  Sybel; 
for  it  summarizes  what  these  and  other  writers  tell  us.  Mr.  Mallet  presents 
economic  and  political  aspects  of  society  before  the  Revolution  ;  attempts  to 
explain  why  the  Revolution  came  ;  why  the  men  who  made  it  failed  to  attain 
the  liberty  they  so  ardently  desired,  or  to  found  the  new  order  which  they  hoped 
to  see  in  France  ;  by  what  arts  and  accidents,  owing  to  what  deeper  causes,  an 
inconspicuous  minority  gradually  grew  into  a  victorious  party  ;  how  external 
circumstances  kept  the  revolutionary  fever  up,  and  forced  the  Revolution  for- 
ward. He  undertakes  to  make  clear  the  mystery  of  the  time,  the  real  character 
and  aims  of  the  men  who  grasped  the  supreme  power  in  1793-4,  who  held  it 
with  such  a  combination  of  energy  and  folly,  of  heroism  and  crime,  and  who 
proceeded,  through  anarchy  and  terror,  to  experiment  how  social  misery  could 
be  extinguished  and  universal  felicity  attained,  by  drastic  philosophic  remedies, 
applied  by  despots,  and  enforced  by  death.  History  offers  no  problem  of  more 
surpassing  interest,  and  none  more  perplexing  or  obscure. 


UNIVERSITY    MANUALS 


LOGIC,  INDUCTIVE    AND    DEDUCTIVE 

By  WILLIAM  Minto,    Late  Professor  of  Logic   and  Literature, 
University  of  Aberdeen.     With  Diagrams.      i2mo,  $  net. 

In  press. 

Contents:  Parti:  I.  General  Names  and  Allied  Distinctions— II.  The  Syllo- 
gistic Analysis  of  Propositions  into  Terms.  Part  II:  I.  Imperfect  Understanding 
of  Words  and  the  Remedies  Therefor;  Dialetic;  Definition — II.  The  Five  Pred- 
icables  ;  Verbal  and  Real  Predication — III.  Aristotle's  Categories — IV.  The 
Controversy  between  Umversals — Difficulties  Concerning  the  Relation  of  Gen- 
eral Names  to  Thought  and  to  Reality.  Part  III:  I.  Theories  of  Predication — 
Theories  of  Judgment — II.  The  "Opposition"  of  Propositions — The  Interpre- 
tation of  "  No  " — III.  The  Implication  of  Propositions — Immediate  Formal 
Inference  Eduction — IV.  The  Counter-Implications  of  Propositions.  Part  IV  : 
I.  The  Syllogism — II.  Figures  and  Moods  of  the  Syllogism — III.  The  Demon- 
stration of  the  Syllogistic  Moods — The  Canons  of  the  Syllogism— IV.  The 
Analysis  of  Arguments  into  Syllogistic  Forms — V.  Euthymemes — VI.  The 
Utility  of  the  Syllogism — VII.  Conditional  Arguments — Hypothetical  Syllo- 
gism— Dis.unctive  Syllogism  and  Dilemma — VIII.  Fallacies  in  Deductive  Argu- 
ment— Pt  .no  Principii  and  Ignoratio  Elenchi. 


CHAPTERS    IN    MODERN    BOTANY 

By  Patrick  Geddes,   Professor  of  Botany,  University  College, 
Dundee.      Illustrations.      i2mo,  %  net. 

Contents:  Land  II.  Pitcher  Plants — III.  Other  Insectivorous  Plants, 
Difficulties  and  Criticisms — IV.  and  V.  Movement  and  Nervous  Action  in 
Plants — VI.  The  Web  of  Life — VII.  Relations  between  Plants  and  Animals 
— VIII.  Spring  and  its  Studies  ;  Geographical  Distribution  and  World  Land- 
scapes ;  Seedling  and  Bud — IX.  Leaves — X.  Suggestions  and  Further  Study. 


THE    PHILOSOPHY    OF   THE    BEAUTIFUL 

Its  Theory  and  Its  Relation  to  the  Arts.      Part  II.     By  Professor 
Knight,  University  of  St.  Andrews.      i2mo,  %  In  press. 

Contents:  I.  Prolegomena — II.  The  Nature  of  Beauty — III.  The  Ideal 
and  the  Real — IV.  Inaacquate  or  Partial  Theories — V.  Suggestions  towards 
a  more  Complete  Theory  of  Beauty — VI.  Art,  its  Nature  and  Functions — 
VII.  The  Correlation  of  the  Arts — VIII.  Poetry — IX.  Music— X.  Archi- 
tecture— XL  Sculpture — XII.  Painting — XIII.  Dancing — Appendix:  a.  Rus- 
sian ^Esthetic — b.  Danish  ^Esthetic. 


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